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Recirculating systems for holding
and growing fish have been used
by fisheries researchers for more
than three decades. Attempts to
advance these systems to com-
mercial scale food fish production
have increased dramatically in the
last decade. The renewed interest
in recirculating systems is due to
their perceived advantages,
including: greatly reduced land
and water requirements; a high
degree of environmental control
allowing year-round growth at
optimum rates; and the feasibility
of locating in close proximity to
prime markets.
Unfortunately, many commercial
systems, to date, have failed
because of poor design, inferior
management, or flawed econom-
ics. This publication will address
the problems of managing a recir-
culating aquaculture system so
that those contemplating invest-
ment can make informed deci-
sions. For information on theory
and design of recirculating sys-
tems refer to SRAC Publication
No. 451, Recirculating Aquaculture
Tank Production Systems: An
Overview of Critical Considerations,
and SRAC Publication No. 453,
Recirculating Aquaculture Tank
Production Systems: Component
Options.
Recirculating systems are mechan-
ically sophisticated and biological-
ly complex. Component failures,
poor water quality, stress, dis-
eases, and off-flavor are common
problems in poorly managed
recirculating systems.
Management of these systems
takes education, expertise and
dedication.
Recirculating systems are biologi-
cally intense. Fish are usually
reared intensively (0.5 pound/gal-
lon or greater) for recirculating
systems to be cost effective. As an
analogy, a 20-gallon home aquari-
um, which is a miniature recircu-
lating system, would have to
maintain at least 10 pounds of fish
to reach this same level of intensi-
ty. This should be a sobering
thought to anyone contemplating
the management of an intensive
recirculating system.
System operation
To provide a suitable environment
for intensive fish production,
recirculating systems must main-
tain uniform flow rates (water and
air/oxygen), fixed water levels,
and uninterrupted operation.
The main cause of flow reduction
is the constriction of pipes and air
diffusers by the growth of fungi,
bacteria and algae, which prolifer-
ate in response to high levels of
nutrients and organic matter. This
can cause increases or decreases in
tank water levels, reduce aeration
efficiency, and reduce biofilter effi-
ciency. Flow rate reduction can be
avoided or mitigated by using
oversized pipe diameters and con-
figuring system components to
shorten piping distances. The
fouling of pipes leaving tanks (by
gravity flow) is easily observed
because of the accompanying rise
in tank water level. If flow rates
gradually decline, then pipes
must be cleaned. A sponge, clean-
ing pad or brush attached to a
plumber’s snake works well for
scouring pipes. Air diffusers
should be cleaned periodically by
soaking them in muriatic acid
(available at plumbing suppliers).
Flow blockage and water level
fluctuations also can result from
the clogging of screens used to
retain fish in the rearing tanks.
Screen mesh should be the largest
size that will retain the fish (usu-
ally
3
/4 to 1 inch). The screened
area around pipes should be
much larger than the pipe diame-
ter, because a few dead fish can
easily block a pipe. Screens can be
made into long cylinders or boxes
that attach to pipes and have a
large surface area to prevent
blockage. Screens should be tight-
March 1999
Revision
SRAC Publication No. 452
Recirculating Aquaculture Tank
Production Systems
Management of Recirculating Systems
Michael P. Masser
1
, James Rakocy
2
and Thomas M. Losordo
3
1
Auburn University;
2
University of the Virgin Islands;
3
North Carolina State University
ly secured to the pipe so that they
cannot be dislodged during feed-
ing, cleaning and harvesting oper-
ations.
An essential component of recir-
culating systems is a backup
power source (see SRAC
Publication No. 453). Electrical
power failures may not be com-
mon, but it only takes a brief
power failure to cause a cata-
strophic fish loss. For example, if
a power failure occurred in a
warmwater system (84
o
F) at sat-
urated oxygen concentrations
containing
1
/2-pound fish at a
density of
1
/4 pound of fish per
gallon of water, it will take only
16 minutes for the oxygen con-
centration to decrease to 3 ppm, a
stressful level for fish. The same
system containing 1-pound fish at
a density of 1 pound of fish per
gallon would plunge to this
stressful oxygen concentration in
less than 6 minutes. These scenar-
ios should give the prospective
manager a sobering feeling for
how important backup power is
to the integrity of a recirculating
system.
Certain components of backup
systems need to be automatic. An
automatic transfer switch should
start the backup generator in case
personnel are not present. Auto-
matic phone alarm systems are
inexpensive and are essential in
alerting key personnel to power
failures or water level fluctua-
tions. Some phone alarm systems
allow in-dialing so that managers
can phone in and check on the
status of the system. Other com-
ponent failures can also lead to
disastrous results in a very short
time. Therefore, systems should
be designed with essential backup
components that come on auto-
matically or can be turned on
quickly with just a flip of a
switch. Finally, one of the sim-
plest backups is a tank of pure
oxygen connected with a solenoid
valve that opens automatically
during power failures. This oxy-
gen-solenoid system can provide
sufficient dissolved oxygen to
keep the fish alive during power
failures.
Biological filters (biofilters) can
fail because of senescence, chemi-
cal treatment (e. g., disease treat-
ment), or anoxia. It takes weeks to
months to establish or colonize a
biofilter. The bacteria that colonize
a biofilter grow, age and die.
These bacteria are susceptible to
changes in water quality (low dis-
solved oxygen [DO], low alkalini-
ty, low or high pH, high CO
2
,
etc.), chemical treatments, and
oxygen depletions. Biological fil-
ters do not take rapid change
well!
Particulates
Particulate removal is one of the
most complicated problems in
recirculating systems. Particulates
come from uneaten feed and from
undigested wastes. It has been
estimated that more than 60 per-
cent of feed placed into the sys-
tem ends up as particulates. Quick
and efficient removal of particu-
lates can significantly reduce the
biological demand placed on the
biofilter, improve biofilter efficien-
cy, reduce the overall size of the
biofilter required, and lower the
oxygen demand on the system.
Particulate filters should be
cleaned frequently and main-
tained at peak efficiency. Many
particulates are too small to be
removed by conventional particu-
late filters and cause or compli-
cate many other system problems.
Water quality management
In recirculating systems, good
water quality must be maintained
for maximum fish growth and for
optimum effectiveness of bacteria
in the biofilter (Fig. 1). Water qual-
ity factors that must be monitored
and/or controlled include temper-
ature, dissolved oxygen, carbon
dioxide, pH, ammonia, nitrite and
solids. Other water quality factors
that should be considered are
alkalinity, nitrate and chloride.
Temperature
Temperature must be maintained
within the range for optimum
growth of the cultured species. At
optimum temperatures fish grow
quickly, convert feed efficiently,
and are relatively resistant to
many diseases. Biofilter efficiency
also is affected by temperature but
is not generally a problem in
warmwater systems. Temperature
can be regulated with electrical
immersion heaters, gas or electric
heating units, heat exchangers,
chillers, or heat pumps. Tempera-
DENITRIFICATION
NITRIFICATION
ION BALANCE
GAS STRIPPING
ALKALINITY
ADDITION
BOD REDUCTION
DISSOLVED
REFRACTORY
MANAGEMENT
AERATION
SOLIDS REMOVAL
NO2
H+
N2
NO3
CO2
O2
TAN
BOD
SOLIDS
INERT
SOLIDS
BACTERIA
RFM 6/6/90
CLOSED RECIRCULATING SYSTEM
Figure 1. Diagram of fish wastes and their effects on bacterial and chemical
interactions in a recirculating system.
Courtesy of Ronald F. Malone, Department of Civil Engineering, Louisiana State University, from
Louisiana Aquaculture 1992,“Design of Recirculating Systems for Intensive Tilapia Culture,”
Douglas G. Drennan and Ronald F. Malone.
ture can be manipulated to reduce
stress during handling and to con-
trol certain diseases (e.g., Ich and
ESC).
Dissolved oxygen
Continuously supplying adequate
amounts of dissolved oxygen to
fish and the bacteria/biofilter in
the recirculating system is essen-
tial to its proper operation.
Dissolved oxygen (DO) concentra-
tions should be maintained above
60 percent of saturation or above 5
ppm for optimum fish growth in
most warmwater systems. It is
also important to maintain DO
concentrations in the biofilter for
maximum ammonia and nitrite
removal. Nitrifying bacteria
become inefficient at DO concen-
trations below 2 ppm.
Aeration systems must operate
continuously to support the high
demand for oxygen by the fish
and microorganisms in the sys-
tem. As fish approach harvest size
and feeding rates (pounds/sys-
tem) are near their maximum lev-
els, oxygen demand may exceed
the capacity of the aeration system
to maintain DO concentrations
above 5 ppm. Fish show signs of
oxygen stress by gathering at the
surface and swimming into the
current produced by the aeration
device (e. g., agitator, air lift, etc.)
where DO concentrations are
higher. If this occurs, a supple-
mental aeration system should be
used or the feeding rate must be
reduced.
Periods of heavy feeding may be
sustained by multiple or continu-
ous feedings of the daily ration
over a 15- to 20-hour period rather
than in two or three discrete
meals. As fish digest food, their
respiration rate increases dramati-
cally, causing a rapid decrease in
DO concentrations. Feeding small
amounts continuously with auto-
matic or demand feeders allows
DO to decline gradually without
reaching critical levels. During
periods of heavy feeding, DO
should be monitored closely, par-
ticularly before and after feedings.
Recirculating systems require con-
stant monitoring to ensure they
are functioning properly.
Water said to be “saturated” with
oxygen contains the maximum
amount of oxygen that will dis-
solve in it at a given temperature,
salinity and pressure (Table 1).
Pure oxygen systems can be incor-
porated into recirculating systems.
These inject oxygen into a con-
fined stream of water, creating
supersaturated conditions (see
SRAC Publication No. 453).
Supersaturated water, with DO
concentrations several times high-
er than saturation, is mixed into
the rearing tank water to maintain
DO concentrations near satura-
tion. The supersaturated water
should be introduced into the
rearing tank near the bottom and
be rapidly mixed throughout the
tank by currents generated from
the water pumping equipment.
Proper mixing of the supersaturat-
ed water into the tank is critical.
Dissolved oxygen will escape into
the air if the supersaturated water
is agitated too vigorously. If the
water is mixed too slowly, zones
of supersaturation can cause gas
bubble disease. In gas bubble dis-
ease, gases come out of solution
inside the fish and form bubbles
in the blood. These bubbles can
result in death. Fry are particular-
ly sensitive to supersaturation.
Carbon dioxide
Carbon dioxide is produced by
respiration of fish and bacteria in
the system. Fish begin to stress at
carbon dioxide concentrations
above 20 ppm because it interferes
with oxygen uptake. Like oxygen
stress, fish under CO
2
stress come
to the surface and congregate
around aeration devices (if pre-
sent). Lethargic behavior and a
sharply reduced appetite are com-
mon symptoms of carbon dioxide
stress.
Carbon dioxide can accumulate in
recirculating systems unless it is
physically or chemically removed.
Carbon dioxide usually is
removed from the water by
packed column aerators or other
aeration devices (see SRAC
Publication No. 453).
pH
Fish generally can tolerate a pH
range from 6 to 9.5, although a
rapid pH change of two units or
more is harmful, especially to fry.
Biofilter bacteria which are impor-
tant in decomposing waste prod-
ucts are not efficient over a wide
pH range. The optimum pH range
for biofilter bacteria is 7 to 8.
The pH tends to decline in recir-
culating systems as bacterial nitri-
fication produces acids and con-
sumes alkalinity, and as carbon
dioxide is generated by the fish
and microorganisms. Carbon
dioxide reacts with water to form
carbonic acid, which drives the
pH downward. Below a pH of 6,
the nitrifying bacteria are inhibit-
ed and do not remove toxic nitro-
gen wastes.
Optimum pH range generally is
maintained in recirculating sys-
tems by adding alkaline buffers.
The most commonly used buffers
are sodium bicarbonate and calci-
um carbonate, but calcium
hydroxide, calcium oxide, and
sodium hydroxide have been
used. Calcium carbonate may dis-
solve too slowly to neutralize a
rapid accumulation of acid.
Table 1. Oxygen saturation levels in fresh water at sea level
atmospheric pressure.
Temperature DO Temperature DO
o
C
o
F mg/L (ppm)
o
C
o
F mg/L (ppm)
10 50.0 10.92 24 75.2 8.25
12 53.6 10.43 26 78.8 7.99
14 57.2 9.98 28 82.4 7.75
16 60.8 9.56 30 86.0 7.53
18 64.4 9.18 32 89.6 7.32
20 68.0 8.84 34 93.2 7.13
22 71.6 8.53 36 96.8 6.95
daily. If total ammonia concentra-
tions start to increase, the biofilter
may not be working properly or
the feeding rate/ammonia nitro-
gen production is higher than the
design capacity of the biofilter.
Calcium hydroxide, calcium oxide
and sodium hydroxide dissolve
quickly but are very caustic; these
compounds should not be added
to the rearing tank because they
may harm the fish by creating
zones of very high pH. The pH of
the system should be monitored
daily and adjusted as necessary to
maintain optimum levels. Usually,
the addition of sodium bicarbon-
ate at a rate of 17 to 20 percent of
the daily feeding rate is sufficient
to maintain pH and alkalinity
within the desired range (Fig. 2).
For example, if a tank is being fed
10 pounds of feed per day then
approximately 2 pounds of bicar-
bonate would be added daily to
adjust pH and alkalinity levels.
Alkalinity, the acid neutralizing
capacity of the water, should be
maintained at 50 to 100 mg as cal-
cium carbonate/L or higher, as
should hardness. Generally, the
addition of alkaline buffers used
to adjust pH will provide ade-
quate alkalinity, and if the buffers
also contain calcium, they add to
hardness. For a more detailed dis-
cussion of alkalinity and hardness
consult a water quality text.
Nitrogen wastes
Ammonia is the principal nitroge-
nous waste released by fish and is
mainly excreted across the gills as
ammonia gas. Ammonia is a
byproduct from the digestion of
protein. An estimated 2.2 pounds
of ammonia nitrogen are pro-
duced from each 100 pounds of
feed fed. Bacteria in the biofilter
convert ammonia to nitrite and
nitrite to nitrate, a process called
nitrification. Both ammonia and
nitrite are toxic to fish and are,
therefore, major management
problems in recirculating systems
(Fig. 2).
Ammonia in water exists as two
compounds: ionized (NH
4
+
) and
un-ionized (NH
3
) ammonia. Un-
ionized ammonia is extremely
toxic to fish. The amount of un-
ionized ammonia present depends
on pH and temperature of the
water (Table 2). Un-ionized
ammonia nitrogen concentrations
as low as 0.02-0.07 ppm have been
shown to slow growth and cause
tissue damage in several species
of warmwater fish. However,
tilapia tolerate high un-ionized
ammonia concentrations and sel-
dom display toxic effects in well-
buffered recirculating systems.
Ammonia should be monitored
Table 2. Percentage of total ammonia in the un-ionized form at
differing pH values and temperatures.
Temperature (
o
C)
pH 16 18 20 22 24 26 28 30 32
7.0 0.30 0.34 0.40 0.46 0.52 0.60 0.70 0.81 0.95
7.2 0.47 0.54 0.63 0.72 0.82 0.95 1.10 1.27 1.50
7.4 0.74 0.86 0.99 1.14 1.30 1.50 1.73 2.00 2.36
7.6 1.17 1.35 1.56 1.79 2.05 2.35 2.72 3.13 3.69
7.8 1.84 2.12 2.45 2.80 3.21 3.68 4.24 4.88 5.72
8.0 2.88 3.32 3.83 4.37 4.99 5.71 6.55 7.52 8.77
8.2 4.49 5.16 5.94 6.76 7.68 8.75 10.00 11.41 13.22
8.4 6.93 7.94 9.09 10.30 11.65 13.20 14.98 16.96 19.46
8.6 10.56 12.03 13.68 15.40 17.28 19.42 21.83 24.45 27.68
8.8 15.76 17.82 20.08 22.38 24.88 27.64 30.68 33.90 37.76
9.0 22.87 25.57 28.47 31.37 34.42 37.71 41.23 44.84 49.02
9.2 31.97 35.25 38.69 42.01 45.41 48.96 52.65 56.30 60.38
9.4 42.68 46.32 50.00 53.45 56.86 60.33 63.79 67.12 70.72
9.6 54.14 57.77 61.31 64.54 67.63 70.67 73.63 76.39 79.29
9.8 65.17 68.43 71.53 74.25 76.81 79.25 81.57 83.68 85.85
10.0 74.78 77.46 79.92 82.05 84.00 85.82 87.52 89.05 90.58
10.2 82.45 84.48 86.32 87.87 89.27 90.56 91.75 92.80 93.84
8.5
8.0
7.5
7.0
6.5
6.0
0 100 200 300 400 500
Discontinue
supplemental aeration
Reduce daily
bicarbonate
addition
Increase
aeration
Add
sodium
bicarbonate
& aerate
Add
sodium
bicarbonate
Optimum
Alkalinity, mg/L as CaCO3
Figure 2.The pH management diagram, a graphical solution of the ionization constant
equation for carbonic acid at 25
o
C.
Courtesy of Ronald F. Malone, Department of Civil Engineering, Louisiana State University, from
Master’s Thesis of Peter A. Allain, 1988,“Ion Shifts and pH Management in High Density Shedding
Systems for Blue Crabs (Callinectes sapidus) and Red Swamp Crawfish (Procambarus clarkii),”
Louisiana State University.
Biofilters consist of actively grow-
ing bacteria attached to some sur-
face(s). Biofilters can fail if the
bacteria die or are inhibited by
natural aging, toxicity from chem-
icals (e. g., disease treatment), lack
of oxygen, low pH, or other fac-
tors. Biofilters are designed so that
aging cells slough off to create
space for active new bacterial
growth. However, there can be sit-
uations (e. g., cleaning too vigor-
ously) where all the bacteria are
removed. If chemical additions
cause biofilter failure, the water in
the system should be exchanged.
The biofilter would then have to
be re-activated (taking 3 or 4
weeks) and the pH adjusted to
optimum levels.
During disruptions in biofilter
performance, the feeding rate
should be reduced considerably
or feeding should be stopped.
Feeding, even after a complete
water exchange, can cause ammo-
nia nitrogen or nitrite nitrogen
concentrations (Fig. 3) to rise to
stressful levels in a matter of
hours if the biofilter is not func-
tioning properly. Subdividing or
compartmentalizing biofilters
reduces the likelihood of a com-
plete failure and gives the manag-
er the option of “seeding” active
biofilter sludge from one tank or
system to another.
Activating a new biofilter (i. e.,
developing a healthy population
of nitrifying bacteria capable of
removing the ammonia and
nitrite produced at normal feed-
ing rates) requires a least 1
month. During this activation
period, the normal stocking and
feeding rates should be greatly
reduced. Prior to stocking it is
advantageous, but not absolutely
necessary, to pre-activate the
biofilters. Pre-activation is accom-
plished by seeding the filter(s)
with nitrifying bacteria (available
commercially) and providing a
synthetic growth medium for a
period of 2 weeks. The growth
medium contains a source of
ammonia nitrogen (10 to 20
mg/l), trace elements and a buffer
(Table 3). The buffer (sodium
bicarbonate) should be added to
maintain a pH of 7.5. After the
activation period the nutrient
solution is discarded.
Many fish can die during this
period of biofilter activation.
Managers have a tendency to
overfeed, which leads to the gen-
eration of more ammonia than the
biofilter can initially handle. At
first, ammonia concentrations
increase sharply and fish stop
feeding and are seen swimming
into the current produced by the
aeration device. Deaths will soon
occur unless immediate action is
taken. At the first sign of high
ammonia, feeding should be
stopped. If pH is near 7 the fish
may not show signs of stress
because little of the ammonia is in
the un-ionized form.
As nitrifying bacteria, known as
Nitrosomonas, become established
in the biofilter, they quickly con-
vert the ammonia into nitrite. This
conversion takes place about 2
weeks into the activation period
and will proceed even if feeding
has stopped. Once again, fish will
seek relief near aeration and mor-
talities will occur soon unless
steps are taken. Nitrite concentra-
tions decline when a second group
of nitrifying bacteria, known as
Nitrobacter, become established.
These problems can be avoided if
time is taken to activate the biofil-
ters slowly.
Nitrite concentrations also should
be checked daily. The degree of
toxicity to nitrite varies with
species. Scaled species of fish are
generally more tolerant of high
nitrite concentrations than species
such as catfish, which are very
sensitive to nitrite. Nitrite nitrogen
as low as 0.5 ppm is stressful to
catfish, while concentrations of
less than 5 ppm appear to cause
little stress to tilapia. Nitrite toxici-
ty causes a disease called “brown
blood,” which describes the blood
color that results when normal
blood hemoglobin comes in con-
tact with nitrite and forms a com-
pound called methemoglobin.
Methemoglobin does not transport
oxygen properly, and fish react as
if they are under oxygen stress.
Fish suffering nitrite toxicity come
to the surface as in oxygen stress,
sharply reduce their feeding, and
Table 3. Nutrient solution for pre-activation of biofilter.
Nutrient Concentration (ppm)
Dibasic ammonium phosphate, (NH
4
)
2
HPO
4
40
Dibasic sodium phosphate, Na
2
HPO
4
40
Sea salts “solids” 40
Sea salts “liquids” 0.5
Calcium carbonate, CaCO
3
250
24
21
18
15
12
9
6
3
0
System
Ammonia - N
Nitrite - N
Concentration, mg/L
as nitrogen
Figure 3.Typical ammonia and nitrite curves showing time delays in establishing
bacteria in biofilters.
Courtesy of Ronald F. Malone, Department of Civil Engineering, Louisiana State University, from
Master’s Thesis of Don P. Manthe, 1982,“Water Quality of Submerged Biological Rock Filters for
Closed Recirculating Blue Crab Shedding Systems,” Louisiana State University.
are lethargic. Nitrite toxicity can
be reduced or blocked by chloride
ions. Usually 6 to 10 parts of chlo-
ride protect fish from 1 part
nitrite nitrogen. Increasing con-
centrations of nitrite are a sign
that the biofilter is not working
properly or the biofilter is not
large enough to handle the
amount of waste being produced.
As with ammonia buildup, check
pH, alkalinity and dissolved oxy-
gen in the biofilter. Reduce feed-
ing and be prepared to flush the
system with fresh water or add
salt (NaCl) if toxic concentrations
develop.
Nitrate, the end product of nitrifi-
cation, is relatively nontoxic
except at very high concentra-
tions (over 300 ppm). Usually
nitrate does not build up to these
concentrations if some daily
exchange (5 to 10 percent) with
fresh water is part of the manage-
ment routine. Also, in many recir-
culating systems some denitrifica-
tion seems to occur within the
system that keeps nitrate concen-
trations below toxic levels.
Denitrification is the bacteria-
mediated transformation of
nitrate to nitrogen gas, which
escapes into the atmosphere.
Solids
Solid waste, or particulate matter,
consists mainly of feces and
uneaten feed. It is extremely
important to remove solids from
the system as quickly as possible.
If solids are allowed to remain in
the system, their decomposition
will consume oxygen and pro-
duce additional ammonia and
other toxic gases (e. g., hydrogen
sulfide). Solids are removed by
filtration or settling (SRAC
Publication No. 453). A consider-
able amount of highly malodor-
ous sludge is produced by recir-
culating systems, and it must be
disposed of in an environmental-
ly sound manner (e. g., applied to
agricultural land or composted).
Very small (colloidal) solids
remain suspended in the water.
Although the decay of this mater-
ial consumes oxygen and pro-
duces some additional ammonia,
it also serves as attachment sites
for nitrifying bacteria. Therefore,
a low level of suspended solids
may serve a beneficial role within
the system as long as they do not
irritate the fishes’ gills.
If organic solids build up to high
levels in the system, they will
stimulate the growth of microor-
ganisms that produce off-flavor
compounds. The concentration of
solids at which off-flavor com-
pounds develop is not known,
but the system water should
never be allowed to develop a
foul or fecal smell. If offensive
odors develop, increase the water
exchange rate, reduce feeding,
increase solids removal, and/or
enlarge biofilters.
Chloride
Adding salt (NaCl) to the system
is beneficial not only for the chlo-
ride ions, which block nitrite toxi-
city, but also because sodium and
chloride ions relieve osmotic
stress. Osmotic stress is caused by
the loss of ions from the fishes’
body fluids (usually through the
gills). Osmotic stress accompanies
handling and other forms of
stress (e. g., poor water quality).
A salt concentration of 0.02 to 0.2
percent will relieve osmotic stress.
This concentration of salt is bene-
ficial to most species of fish and
invertebrates. It should be noted
that rapidly adding salt to a recir-
culating system can decrease
biofilter efficiency. The biofilter
will slowly adjust to the addition
of salt but this adjustment can
take 3 to 4 weeks. Table 4 summa-
rizes general water quality
requirements of recirculating sys-
tems.
Water exchange
Most recirculating systems are
designed to replace 5 to 10 per-
cent of the system volume each
day with new water. This amount
of exchange prevents the build-up
of nitrates and soluble organic
matter that would eventually
cause problems. In some situa-
tions, sufficient water may not be
available for these high exchange
rates. A complete water exchange
should be done after each produc-
tion cycle to reduce the build-up
of nitrate and dissolved organics.
For emergency situations it is rec-
ommended that the system have
an auxiliary water reservoir equal
to one complete water exchange
(flush). The reservoir should be
maintained at the proper temper-
ature and water quality.
Fish production
management
Stocking
Fish management starts before the
fish are introduced into the recir-
culating system. Fingerlings
should be purchased from a rep-
utable producer who practices
genetic selection, knows how to
carefully handle and transport
fish, and does not have a history
Table 4. Recommended water quality requirements of recirculating
systems.
Component Recommended value or range
Temperature optimum range for species cultured - less
than 5
o
F as a rapid change
Dissolved oxygen 60% or more of saturation, usually 5 ppm
or more for warmwater fish and greater than
2 ppm in biofilter effluent
Carbon dioxide less than 20 ppm
pH 7.0 to 8.0
Total alkalinity 50 to 100 ppm or more as CaCO
3
Total hardness 50 to 100 ppm or more as CaCO
3
Un-ionized ammonia-N less than 0.05 ppm
Nitrite-N less than 0.5 ppm
Salt 0.02 to 0.2 %
of disease problems in his/her
hatchery. Starting with poor quali-
ty or diseased fingerlings almost
ensures failure.
Fish should be checked for para-
sites and diseases before being
introduced into the system. New
fish may need to be quarantined
from fish already in the system so
that diseases will not be intro-
duced. A few fish should be
checked for parasites and diseases
by a certified fish diagnostician.
Once diseases are introduced into
a recirculating system they are
generally hard to control, and
treatment may disrupt the biofil-
ter.
Fish are usually hauled in cool
water. As they come into the sys-
tem they usually have to be tem-
pered or gradually acclimated to
the system temperature and pH.
Fish can generally take a 5
o
F
change without much problem.
Temperature changes of more
than 5
o
F should be done at about
1
o
F every 20 to 30 minutes. Stress
can be reduced if the system is
cooled to the temperature of the
hauling water and then slowly
increased over a period of several
hours to days.
Recirculating systems must oper-
ate near maximum production
(i. e., maximum risk) capacity at
all times to be economical. It is not
cost effective to operate pumps
and aeration devices when the
system is stocked with fingerlings
at only one-tenth of the system’s
carrying capacity. Therefore, fin-
gerlings should be stocked at very
high rates, in the range of 30 fish
per cubic foot. Feeding rates
should be optimum for rapid
growth and near the system maxi-
mum—the highest feeding rates at
which acceptable water quality
conditions can be maintained.
When more feed is required, fish
stocks should be split and moved
to new tanks. This would gradual-
ly reduce the stocking rate over
the production cycle.
Another approach is to divide the
rearing tank(s) into compartments
with different size groups of fish
in each compartment. In this
approach, the optimum feeding
rate for all the compartments is
consistently near the biofilter’s
maximum performance. As one
group of fish is harvested, finger-
lings are immediately stocked into
the vacant compartment or tank.
Compartment size within a tank
may be adjusted as fish grow, by
using movable screens.
Feeding
Knowing how much to feed fish
without overfeeding is a problem
in any type of fish production.
Feeding rates are usually based on
fish size. Small fish consume a
higher percent of their body
weight per day than do larger fish
(Table 5). Most fish being grown
for food will be stocked as finger-
lings. Fingerlings consume 3 to 4
percent of their body weight per
day until they reach
1
/4 to
1
/2
pound, then consume 2 to 3 per-
cent of their body weight until
being harvested at 1 to 2 pounds.
A rule-of-thumb for pond culture
is to feed all the fish will consume
in 5 to 10 minutes. Unfortunately,
this method can easily lead to
overfeeding. Overfeeding wastes
feed, degrades water quality, and
can overload the biofilter.
Table 6 approximates a feeding
schedule for a warmwater fish
(e.g., tilapia) stocked into an 84
o
F
recirculating system as fry and
harvested at a weight of 1 pound
after 250 feeding days. Feed con-
version is estimated at 1.5: 1, or
1.5 pounds of feed to obtain 1
pound of gain.
Tables 5 and 6 are estimates and
should be used only as guidelines
which can change with differing
species and temperatures.
Growth and feed conversion are
estimated by weighing a sample
of fish from each tank and then
calculating the feed conversion
ratios and new feeding rates from
this sample. For example, 1,000
fish in a tank have been consum-
ing 10 pounds of feed a day for
the last 10 days (100 pounds
total). The fish were sampled 10
days earlier and weighed an aver-
age of 0.33 pounds or an estimat-
ed total of 330 pounds.
Table 5. Estimated food con-
sumption by size of a
typical warmwater fish.
Average Body weight
weight per fish consumed
(lbs.) (g) (%)
0.02 9 5.0
0.04 18 4.0
0.06 27 3.3
0.25 113 3.0
0.50 227 2.75
0.75 340 2.5
1.0 454 2.2
1.5 681 1.8
Table 6. Recommended stocking and feeding rates for different size groups of tilapia in tanks, and
estimated growth rates.
Stocking rate Weight (g) Growth rate Growth period Feeding rate
(number/ft3) Initial Final (g/day) (days) (%)
225 0.02 0.5-1 - 30 20 - 15
90 0.5-1 5 - 30 15 - 10
45 5 20 0.5 30 10 - 7
28 20 50 1.0 30 7 - 4
14 50 100 1.5 30 4 - 3.5
5.5 100 250 2.5 30 3.5 - 1.5
3 250 450 3.0 70 1.5 - 1.0
A new sample of 25 fish is collect-
ed from the tank and weighed.
The 25 fish weigh 10 pounds or an
average of 0.4 pounds per fish. If
this is a representative sample,
then 1,000 fish should weigh 400
pounds. Therefore, the change in
total fish weight for this tank is
400 minus 330, or 70 pounds. The
fish were fed 100 pounds of feed
in the last 10 days and gained 70
pounds in weight. Feed conver-
sion then is equal to 1.43 to 1 (i.e.,
100 ÷ 70). In other words, the fish
gained 1 pound of weight for each
1.43 pounds of feed fed. The daily
feeding rate should now be
increased to adjust for growth of
the fish.
To calculate the new feeding rate,
multiply the estimated total fish
weight (400 pounds) by the esti-
mated percent body weight of
feed consumption for a 0.4-pound
fish (from Table 5). Table 5 sug-
gests that the percent body weight
consumed per day should be
between 2.75 and 3 percent. If 3
percent is used, then 400 times
0.03 is 12.0. Thus, the new feeding
rate should be 12 pounds of feed
per day for the next 10 days, for a
total of 120 pounds. Using this
sampling technique the manager
can accurately track growth and
feed conversion, and base other
management decisions on these
factors.
Feeding skills
Feeding is the best opportunity to
observe overall vitality of the fish.
A poor feeding response should
be an immediate alarm to the
manager. Check all aspects of the
system, particularly water quality,
and diagnose for diseases if feed-
ing behavior suddenly diminish-
es.
Fish can be fed once or several
times a day. Multiple feedings
spread out the waste load on the
biofilter and help prevent sudden
decreases in DO. Research has
shown that small fish will grow
faster if fed several times a day.
Feeding several times a day seems
to reduce problems of feeding
dominance in some species of fish.
Many recirculating system man-
agers feed as often as every 30
minutes. Multiple feedings at the
same location in a tank can
increase dominance because a few
fish jealously guard the area and
do not let other fish feed. In this
situation, use feeders that distrib-
ute feed widely across the tank.
Fish can be fed by hand, with
demand feeders, or by automatic
feeders, but stationary demand
and belt type feeders tend to
encourage dominance. Whichever
method is used, be careful to
evenly distribute feed and not to
overfeed.
Always purchase high quality
feed from a reputable company.
Keep feed fresh by storing it in a
cool, dry place. Never use feed
that is past 60 days of the manu-
facture date. Never feed moldy,
discolored or clumped feed.
Molds on feed may produce afla-
toxins, which can stress or kill
fish. Feed quality deteriorates
with time, particularly when
stored in warm, damp conditions.
A disease known as “no blood” is
associated with feed that is defi-
cient in certain vitamins. In a case
of “no blood,” the fish appear
pale with white gills and blood
appears clear, not red. Another
nutritional disease known as “bro-
ken back syndrome” is caused by
a vitamin C deficiency. The only
management practice for “no
blood” disease and “broken back
syndrome” is to discard the feed
being used and purchase a differ-
ent batch or brand of feed.
Fines, crumbled feed particles, are
not generally consumed by the
fish but add to the waste load of
the system, increasing the burden
on particulate and biological fil-
ters. Therefore, it is recommended
that feed pellets be sifted or
screened to remove fines before
feeding.
Off-flavor
Off-flavor in recirculating systems
is a common and persistent prob-
lem. Many times fish have to be
moved into a clean system, one
with clear, uncontaminated water,
where they can be purged of off-
flavor before being marketed.
Purging fish of off-flavor can take
from a few days to many weeks
(depending on the type and sever-
ity of off-flavor). If fish remain in
the purging tanks for an extended
period, their feeding rate may
need to be reduced, or off-flavor
may develop within the purging
system.
See SRAC Publication No. 431,
Testing Flavor Quality of Preharvest
Channel Catfish, for detailed infor-
mation on off-flavor.
Stress and disease control
The key to fish management is
stress management. Fish can be
stressed by changes in tempera-
ture and water quality, by han-
dling (including seining and haul-
ing), by nutritional deficiencies,
and by exposure to parasites and
diseases. Stress increases the sus-
ceptibility of fish to disease, which
can lead to catastrophic fish losses
if not detected and treated quick-
ly. To reduce stress fish must be
handled gently, kept under proper
water quality conditions, and pro-
tected from exposure to poor
water quality and diseases. Even
sound and light can stress fish.
Unexpected sounds or sudden
flashes of light often trigger an
escape response in fish. In a tank,
this escape response may send
fish into the side of the tank, caus-
ing injury. Fish are generally sen-
sitive to light exposure, particular-
ly if it is sudden or intense. For
this reason many recirculating
systems have minimal lighting
around the fish tanks.
Diseases
There are more than 100 known
fish diseases, most of which do
not seem to discriminate between
species. Other diseases are very
host specific. Organisms known to
cause diseases and/or parasitize
fish include viruses, bacteria,
fungi, protozoa, crustaceans, flat-
worms, roundworms and seg-
mented worms. There are also
non-infectious diseases such as
brown blood, no blood and bro-
ken back syndrome. Any of these
diseases can become a problem in
a recirculating system. Diseases
can be introduced into the system
from the water, the fish, and the
system’s equipment.
Diseases are likely to enter the
system from hauling water, on the
fish themselves, or on nets, bas-
kets, gloves, etc., that are moved
from tank to tank. Hauling water
should never be introduced into
the system. Fish should be quar-
antined, checked for diseases, and
treated as necessary. Equipment
should be sterilized (e. g., chlorine
dip) before moving it between
tanks. If possible, provide sepa-
rate nets and baskets for each tank
so they will not contaminate other
tanks. Disease can spread rapidly
from one tank to another if equip-
ment is freely moved between
tanks or if all the water within the
system is mixed together as in a
common sump, particulate filter
or biofilter.
A manager needs to be familiar
with the signs of stress and dis-
ease which include:
■ Excitability
■ Flashing or whirling
■ Skin or fin sores or discol-
orations
■ Staying at the surface
■ Erratic swimming
■ Reduction in feeding rate
■ Gulping at the surface
■ Cessation of feeding
■ Mortalities
Whenever any of these symptoms
appear the manager should check
water quality and have a few fish
with symptoms diagnosed by a
qualified fish disease specialist.
The most common diseases in
recirculating systems are caused
by bacteria and protozoans. Some
diseases that have been particular-
ly problematic in recirculating
systems include the protozoal dis-
eases Ich (Ichthyophthirius) and
Trichodina, and the bacterial dis-
eases columnaris, Aeromonas,
Streptococcus and Mycobacterium. It
appears that Trichodina and
Streptococcus diseases are prob-
lematic in recirculating systems
with tilapia, while Mycobacterium
has been found in hybrid striped
bass in intensive recirculating sys-
tems.
It may be possible to treat dis-
eases with chemicals approved for
fish (see SRAC Publication No.
410, Calculating Treatments for
Ponds and Tanks), although few
therapeutants are approved for
use on food fish species other
than catfish and rainbow trout.
Treatment always has its prob-
lems. In the case of recirculating
systems, chemical treatments can
severely disrupt the biofilter.
Biofilter bacteria are inhibited to
some degree by formalin, copper
sulfate, potassium permanganate,
and certain antibiotics. Even sud-
den changes in salt concentration
will decrease biofilter efficiency. If
the system is designed properly, it
may be possible to isolate the
biofilter from the rest of the sys-
tem, treat and flush the fish tanks,
and then reconnect the biofilter
without exposing it to chemical
treatment. However, there is a
danger that the biofilter will re-
introduce the disease organism.
Whenever a chemical treatment is
applied, be prepared to exchange
the system water and monitor the
DO concentration and other water
quality factors closely. Fish usual-
ly reduce their feed consumption
after a chemical treatment; there-
fore, feeding rates need to be
monitored carefully.
Tables 7 and 8 give possible caus-
es and management options based
on the observation of the fish or
water quality tests.
Conclusions
Recirculating systems have devel-
oped to the point that they are
being used for research, for orna-
mental/tropical fish culture, for
maturing and staging brood fish,
for producing advanced fry/fin-
gerlings, and for producing food
fish for high dollar niche markets.
They continue to be expensive
ventures which are as much art as
science, particularly when it
comes to management. Do your
homework before deciding to
invest in a recirculating system.
Investigate the efficiency, compati-
bility and maintenance require-
ments of the components.
Estimate the costs of building and
operating the system and of mar-
keting the fish without any return
on investment for at least 2 years.
Know the species you intend to
grow, their environmental require-
ments, diseases most common in
their culture, and how those dis-
eases are treated. Know your
potential markets and how the
fish need to be prepared for that
market. Be realistic about the
Examples of fish diseases
A–Columnaris
B–
Aeromonas
C–
Streptococcus
(cataract and pop-eye)
D–
Mycobacterium
(granular liver and spleen)
Table 7. Possible options in managing a recirculating tank system based on observations of the fish.
Observation Possible cause Possible management
Fish:
Excitable/darting/erratic swimming ■ excess or intense reduce sound level/pad sides of tank/reduce
sounds/lights light intensity
■ parasite examine* fish with symptoms
■ high ammonia check ammonia concentration
Flashing/whirling ■ parasite examine fish with symptoms
Discolorations/sores ■ parasite/bacteria examine fish with symptoms
Bloated/eyes bulging out
■ virus or bacteria examine fish with symptoms
■ gas bubble disease check for supersaturation and examine fish
with symptoms
Lying at surface/not swimming off ■ parasite examine fish with symptoms
■ low oxygen check dissolved oxygen in tank
■ high ammonia or nitrite check ammonia and nitrite concentrations
■ bad feed check feed for discoloration/clumping and
check blood of fish
■ high carbon dioxide check carbon dioxide level
Crowding around water inflow/aerators
■ low oxygen check dissolved oxygen in tank
■ parasite/disease examine fish with symptoms
■ high ammonia or nitrite check ammonia and nitrite concentrations
■ bad feed check feed for discoloration/clumping and
check blood of fish
Gulping at surface ■ low oxygen check dissolved oxygen in tank
■ parasite/disease examine fish with symptoms
■ high ammonia or nitrite check ammonia and nitrite concentrations
■ high carbon dioxide check carbon dioxide level
■ bad feed check feed for discoloration/clumping and
check blood of fish
Reducing feeding ■ low oxygen check dissolved oxygen in tank
■ parasite/disease examine fish with symptoms
■ high ammonia or nitrite check ammonia and nitrite concentrations
■ bad feed check feed for discoloration/clumping and
check blood of fish
Stopping feeding ■ low oxygen check dissolved oxygen in tank
■ parasite/disease examine fish with symptoms
■ high ammonia or nitrite check ammonia and nitrite concentrations
Discolored blood –
■ high nitrite examine fish with symptom; add 5 to 6 ppm
Brown chloride for each 1 ppm nitrite; purchase
new feed and discard old feed
Clear (no blood)
■ vitamin deficiency examine fish with symptom; check feed for
discoloration/clumping; purchase new feed
and discard old feed
Broken back or “S” shaped backbone ■ vitamin deficiency examine fish with symptom; purchase new
feed and discard old feed
*Have fish examined by a qualified fish diagnostician.
Table 8. Possible management options based on water quality and feed observations.
Observation Possible management
Low dissolved oxygen (less than 5 ppm) ■ increase aeration
■ stop feeding until corrected
■ watch for symptoms of new parasite/disease
High carbon dioxide (above 20 ppm)
■ add air stripping column
■ increase aeration
■ watch for symptoms of new paraside/disease
Low pH (less than 6.8)
■ add alkaline buffers (sodium bicarbonate, etc.)
■ reduce feeding rate
■ check ammonia and nitrite concentarations
High ammonia (above 0.05 ppm as un-ionized)
■ exchange system water
■ reduce feeding rate
■ check biofilter, pH, alkalinity, hardness, and dissolved oxygen
in the biofilter
■ watch for symptoms of new parasite/disease
High nitrite (above 0.5 ppm)
■ exchange system water
■ reduce feeding rate
■ add 5 to 6 ppm chloride per 1 ppm nitrite
■ check biofilter, pH, alkalinity, hardness, and dissolved oxygen
in the biofilter
■ watch for symptoms of new parasite/disease
Low alkalinity ■ add alkaline buffers
Low hardness ■ add calcium carbonate or calcium chloride
Discolored/clumped feed
■ purchase new feed and discard old feed
■ watch for symptoms of new parasite/disease
money, time and effort you are
willing to invest while you are in
the learning curve of managing a
recirculating system.
Finally, design the system with an
emergency aeration system, back-
up power sources, and backup
system components. Monitor
water quality daily and maintain
it within optimum ranges.
Exclude diseases at stocking.
Perform routine diagnostic checks
and be prepared to treat diseases.
Reduce stress whenever and how-
ever possible. STAY ALERT!
The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045
from the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.
