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How Many Fish Can I Keep?

How to Determine the Safe Holding Capacity of Your Fish Tank

(The First Tank Guide)

People often ask about the safe population of their fish tank or fish bowl. They want to know how many fish they can keep in their tank - or they have been told all kinds of weird things about aquarium population and tank capacity and they can't make sense of it.

There are several methods aquariists use to determine the safe holding 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.

This table shows the estimated total number of the specified fish the tank can house safely according to the calculation method on that row, assuming there are no other fish or other animals in the tank, and the tank is cycled and appropriately filtered and maintained.
Fish Size (inches) Weight (grams - est.) Number of Fish by Tank Size
method 2.5 gal. 5 gal. 10 gal. 20 gal. 29 gal. 55 gal. 180 gal.
Neon Tetra 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./ 3.00 4.67 8.33 12.00 15.00 26.00 72.00
Freshwater Angelfish 6 20 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./ 1.00 1.56 2.78 4.00 5.00 8.67 24.00
Common Plecostomus (large and messy fish) 24 700 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./ 0.08 0.13 0.23 0.32 0.42 0.72 2.00
Fancy Goldfish 8 200 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./ 0.75 1.17 2.08 3.00 3.75 6.50 18.00
Oscar (large and messy fish) 12 500 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
in./ 0.17 0.26 0.46 0.67 0.83 1.44 4.00
Tinfoil Barb (large fish) 16 250 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./ 0.12 0.19 0.35 0.50 0.62 1.08 3.00
Tiger Barb 3 3 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./ 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./ 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 holding capacity (or maximum tank population) of your aquarium will vary depending on many factors, and these guidelines are based on several of those factors. These are all primarily related to the distribution of oxygen to the fish, biological filter, and other organisms in the aquarium and the dilution of water products in the water so that the filter can process it before the fish are harmed.

Filtration and Aquarium Population

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 wet-dry 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.

Waste Dissipation in Aquarium Water

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.

Oxygen Availability and Aquarium Population

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.

Aquarium Maintenance and Tank Population

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.

Fish Size and Aquarium Population

Remember, when you are calculating capacity of your tank, ALWAYS 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.

Aquarium Overpopulation

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.

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