How Many Fish Can I Keep?
How to Determine the Safe Holding Capacity of Your Fish
Tank
People often ask about the capacity of their tank or bowl. There are several methods
aquariists use to determine the capacity of their aquarium. One inch of fish per gallon
of water is a good rule of thumb for most small community fish (like most Tetras,
Bettas, Rainbowfish, and Platies). However, larger species (like Oscars, Channel
Catfish, Plecostomus, and Tinfoil Barbs), or messier species (like Oscars, Piranhas,
Koi, and African Cichlids), marine fish (like Blue Damsels, Klein's Butterflies,
Bangai Cardinalfish, and Percula Clowns), and colder water fish (like Goldfish,
Dragonfish, Gars, and Bass) need more available resources, so they should only
have one inch of fish for every three or more gallons of water. Other methods
calculate fish weight per volume of water (1 gram of fish for every 4 liters of water),
fish length to filter capacity, or fish length to surface area (1" of fish for every 12 sq.
in. of surface). However, all of these methods will vary as noted above for large or
messy fish. Additionally, there are specific types of fish that have more demanding
needs or are more adamant about territories and space, and the needs of these fish
will need to be taken into consideration if you are keeping any of them. As shown in
the chart below, the results each of these rules will give you can vary considerably,
especially in larger tanks.
Fish
Neon Tetra
Estimated Number of Fish by Tank Size
Weight
Size
(grams 2.5 5
10
20
29
55
180
(inches)
approximated) method gal. gal. gal. gal. gal. gal.
gal.
2
1
in./gal.
1.25 2.50 5.00 10.00 14.50 27.50 90.00
g/l
2.37 4.74 9.48 18.95 27.48 52.11 170.55
in./sq.in. 3.00 4.67 8.33 12.00 15.00 26.00 72.00
Freshwater
Angelfish
Common
Plecostomus
(large and
messy fish)
Fancy
Goldfish
Oscar
(large and
6
24
8
12
20
700
200
500
in./gal.
0.42 0.83 1.67 3.33
4.83
9.17
30.00
g/l
0.12 0.24 0.47 0.95
1.37
2.61
8.53
in./sq.in. 1.00 1.56 2.78 4.00
5.00
8.67
24.00
in./gal.
0.03 0.07 0.14 0.28
0.40
0.76
2.50
g/l
0.00 0.00 0.00 0.01
0.01
0.02
0.08
in./sq.in. 0.08 0.13 0.23 0.32
0.42
0.72
2.00
in./gal.
0.31 0.62 1.25 2.50
3.62
6.88
22.50
g/l
0.01 0.02 0.05 0.09
0.14
0.26
0.85
in./sq.in. 0.75 1.17 2.08 3.00
3.75
6.50
18.00
in./gal.
0.07 0.14 0.28 0.56
0.81
1.53
5.00
g/l
0.00 0.00 0.01 0.01
0.02
0.03
0.11
mesy fish)
Tinfoil Barb
(large fish)
Tiger Barb
16
3
250
3
in./sq.in. 0.17 0.26 0.46 0.67
0.83
1.44
4.00
in./gal.
0.05 0.10 0.21 0.42
0.60
1.15
3.75
g/l
0.00 0.01 0.01 0.03
0.04
0.07
0.23
in./sq.in. 0.12 0.19 0.35 0.50
0.62
1.08
3.00
in./gal.
0.83 1.67 3.33 6.67
9.67 18.33 60.00
g/l
0.79 1.58 3.16 6.32
9.16 17.37 56.85
in./sq.in. 2.00 3.11 5.56 8.00 10.00 17.33 48.00
Swordtail
4
2.5
in./gal.
0.62 1.25 2.50 5.00
7.25 13.75 45.00
g/l
0.95 1.90 3.79 7.58 10.99 20.84 68.22
in./sq.in. 1.50 2.33 4.17 6.00 7.50 13.00 36.00
Each of the guidelines above are just guidelines, they are not hard and fast rules,
though following one will give you a god idea of what your tank will support. I
prefer to calculate based on the 'inches of fish per gallon of water' rule, because it is
relatively easy to figure out, and gives consistent, reliable results. The actual capacity
of your tank will vary depending on many factors, and these guidelines are based on
several of those factors. First, how much waste can the biological elements of the
filter system in use process. Different filter types and different brands and models
will be able to process different densities of fish waste. Power filters, even those
with an attached rotating drum filter, just cannot handle the volume of fish waste
and decomposing organic matter that a well maintained under gravel filter can
handle, and an under gravel filter can only handle a fraction of what a good wetdry or trickle filter can handle. Canister filters will typically handle more waste
than a power filter, but rarely as much as a trickle filter, and the capacity of these
filters varies greatly depending on the make and model of the canister filter, as well
as the media used in the filter. Another factor that these guidelines are based on is
the ability of the water to dissipate the waste as the fish release it. In this case, a
larger tank volume will keep the water cleaner, without regard for the filtration
system in use. Coupling this with a good filter system and regular water changes
will improve your fishes' health and heartiness. A third factor is dissolved oxygen
available in the water. In this case, the greater the surface area, the greater the
holding capacity of the tank. Any filter that will disturb the surface of the water, such
as power filters or most canister filters, will therefore increase the theoretical load
of the tank, provided that the filters have a good flow rate and are sufficient for the
size of tank in question. Also, along these lines, a trickle filter will provide massive
increases in surface area in the water, getting the solution of atmospheric gases in the
water closer to the proportions in the air in the room. Of course, the biological filter
also requires oxygen to function, so your biological filter will often be more efficient
in a well aerated tank, but a fair portion of the oxygen dissolved in the water may be
taken up by the filter, and not really leave any extra for additional fish. However,
even multiple filters on your tank will not greatly increase the holding capacity of
the tank.
In addition to these concerns, the maintenance provided to the aquarium and the
equipment can greatly influence the tank capacity. Providing good maintenance,
including regular, frequent, small water changes, filter cleaning and filter
cartridge replacement (if necessary), cannot increase the capacity of the tank, but
failing to do this will certainly decrease the capacity.
Remember, when you are calculating capacity of your tank, consider the adult size
of the fish in question. Almost all fish available in a pet store are babies or, at best,
juveniles, and they frequently will have quite a bit of growing to do once you get
them home. This should be accounted for in space available in the current tank or in
anticipated and planned upgrades to larger tanks. For most fish, information on
their typical adult size is available on the fish labels on tanks in the pet store. For
example, freshwater Angel Fish, a 6" fish, should probably have about 6 gallons
each, while Oscars, a 12" messy fish should have about 30 gallons each. Another
factor to consider when determining whether a fish can safely be housed in your
aquarium is the adult size of the fish when compared to the dimensions of the
tank. Most books on fish keeping will list the mature or adult size for most of the
fish the author covers. Frequently, these books will also give a recommendation for a
minimum tank size, either in volume (liters or gallons) or in length (in inches or
centimeters), or , if you're lucky, in both to help you plan your aquarium
appropriately.
Overpopulation in a tank will lead to or contribute to many problems. Aggression is
frequently seen in tanks with too many fish for the available hiding places or
territories. If fish are unable to establish a territory they think is sufficient, they will
have a greater tendency to be aggressive toward other fish in the tank. Harsher
competition for food is also caused by overpopulation. When the fish are afraid that
they will not get their share of the available food at feeding time, they can become
aggressive, and can also overeat, causing other problems down the road. An
overpopulated tank will require greater maintenance, and fish waste builds up faster
and the filter gets dirty. Also, an overpopulated tank will frequently produce more
waste than the filter can efficiently process, allowing the waste in the tank to build to
potentially toxic levels, even if the filter is frequently cleaned. Higher stress, related
to greater aggression and poor water quality will lead to a higher rate of disease and
shorter life spans. Poorer water quality will also contribute substantially to algae
growth, in fact, most algae problems in aquariums can be traced to poor water
quality caused by overpopulation.
Though a larger tank is always better for your fish, for your checkbook, and for your
free time, there are fish that can be kept in small tanks. These include Rasbora Hets,
White Clouds, Neon Tetras, Bettas, and Guppies. Again, any of these fish will do
much better in larger tanks, and the larger tank will give you room to add more fish,
reduce tank maintenance, and provide a happier, healthier life for your fish.
| How Many Fish Can I Keep? | | How Big Will Fish Get? |
| How Much Space Do Fish Need? |
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"[This] site is the first site, which really explains
"Thanks for all the information. I'm
[fish care] with a lot of patience and knowledge
setting up my 1st freshwater fish tank
and I wish I had read your advice before setting and your info helped me out alot.
up my sons first tank. "
Thanks again!"
Pat, Anna Maria, FL
Brooke Beach
March 12th, 2003
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March 20th, 2001
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My snail mail address is: Keith Seyffarth, 217 W. Koch, #308, Bozeman, MT 59715.
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Page most recently modified: July 11 2005 19:41:48.
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