How many fish can you keep in your tank?

Don't crowd your aquarium, it can only hold so many fish

There are guidelines about how many fish you can keep in an aquarium, but few experts would claim they are scientifically exact. The two main rules of thumb on stocking an aquarium are based respectively on the surface area of the tank (which influences oxygen levels) and the tank volume (this takes pollution as well as oxygen into account).

Many other things can alter how many fish an aquarium can healthily support, including:

• The shape of the aquarium
• The filtration system employed
• Surface agitation
• Water temperature
• Tank maintenance regime
• The kinds of fish being kept

Note: You can’t stock to the maximum capacity on first setting up! If your tank can hold, say, 20 small tropical fish, you should build up to that over a period of 3-6 months, to give the tank a chance to mature without a dangerous spike in pollutants.

With those caveats in place, let’s consider the common guidelines.

Fish cm per liter / fish inches per gallon

This was the rule I was taught when I first started keeping fish, and it’s still the most common stocking guide today.

1. Calculate the volume of your aquarium in liters or gallons
2. Work out the length of your chosen fish from nose to base of the tail
3. Consult the table below

Kind of fish       Inches / gallon     cm / liter
Coldwater            1" / gallon          2.5cm / 4.55 liters
Tropical             1" / 0.5 gallon      2.5cm / 2.25 liters
Marine (reef)        1" / 4 gallon        2.5cm / 18 liters
Marine (Fish-only)   1" / 2 gallons       5cm / 9 liters

For example, a 30-gallon aquarium could hold 30″ of coldwater fish, 60″ of tropical fish, or 15″ of marine fish.

Important: I am using Imperial (UK) gallons here. To work out US gallons, multiply UK gallons by 0.83 (or just stick to liters!)

The advantages of the fish inches per gallon rule are that it is easy to understand, since most people know their tank’s volume, and it takes into account the total volume of the tank. “The solution to pollution is dilution” is an old fishkeeping phrase. Bigger volumes of water can obviously carry more fish waste such as nitrate before there’s a problem.

The disadvantage of the rule is it does not take into account the tank shape. A tall, thin tank of 40-gallons will, according to this rule, have the same carrying capacity as a shallower, longer tank. But in reality the shallow tank will offer a much bigger surface area for gas exchange. In filter-less tanks particularly, it makes a big difference.

Fish length per unit of surface area

This rule puts aquarium surface area to the fore:

1. Calculate the surface area of the tank in square inches or centimeters
2. Work out the length of your chosen fish from nose to base of the tail
3. Consult the table below

Kind of fish    Inches / sq inches      cm / sq cm
Coldwater         1" / 30 sq inches        2.5cm / 190 sq cm
Tropical          1" / 12 sq inches        2.5cm / 75 sq cm
Marine            1" / 48 sq inches        2.5cm / 300 sq cm

A 36-inch long by 12-inch wide tank will therefore hold 14″ of coldwater fish, 36″ of tropical fish or 9″ of marine fish.

The advantage of using the surface area guideline is that it takes oxygen exchange directly into account. The ‘per gallon’ rule also does (that’s why coldwater fish need more gallons than tropicals) but not so directly.

The disadvantage of the rule is that people aren’t so aware of their tank’s surface area. It’s easy to work out: just multiply the width of the tank by the length. But it’s harder to keep in mind when you’re in the aquarium store wondering if you can add a few more fish.

Other issues with stocking levels

This article could run forever, so I’ll just add a few more caveats and pointers you should take into account.

Water displacement because of decoration means your tank won’t hold as much as its theoretical volume. Take off at least 10% to account for gravel, rocks, lots of bogwood and so on. With a very heavily furnished tank, take off 15%.

Different fish need a different amounts of space. Active fish like tangs or danios require more swimming room then relatively static fish like clownfish or neon tetras. Many fish require a certain-sized territory. For instance, I’d only keep one pair of dwarf cichlids in a two-foot freshwater aquarium. These measures have nothing to do with theoretical volume.

Big fish need more room than the equivalent number of little fish. Two six-inch long catfish will require a much bigger tank than 12 one-inch tetras. Their biological load is greater, and they’re messier eaters.

You can generally ignore shrimps, snails, plants and invertswhen making your calculations, as they have a small biological load and often contribute to the tank’s carrying capacity (such as plants which oxygenate by day or critters that clean up detritus). Two caveats: In large numbers they will make a difference (such as a snail infestation in a tropical tank), and marine inverts can be more sensitive to poorer water quality (hence the lower stocking suggestion per gallon above).

Strong water circulation, big filters, airstones and so on can increase the stocking level of your aquarium, but I wouldn’t recommend it. Better to have healthy fish with enough room to roam unmolested than to cram in another fish or two and risk disease and infighting.

Remember, these are just rules of thumb. Erring on the side of caution is always the best policy with aquariums, and an under-stocked tank is far easier to keep in good condition than even a slightly over-stocked one.