How many fish an aquarium can hold

It is the question everyone asks when setting up a tank: how many fish can I keep? There is no exact answer, but there is a widely used rule of thumb that gives an order of magnitude: about one centimetre of adult fish per litre of water, for small, slender fish. This calculator applies it to the real volume of your tank and tells you what percentage of the indicative capacity the fish you have in mind would reach.

How the calculation works

• Net volume: the stated tank volume minus the space taken by substrate, rocks and wood, often 10-20%.
• Indicative capacity: net volume divided by the litres per centimetre of fish you choose.
• Load: the total centimetres of planned adult fish divided by the capacity, as a percentage.

Add up the adult size of each fish, not its size in the shop: many fish are sold young and double in length.

Which litres-per-centimetre value to use

• 1 litre per cm: small, slender tropical fish such as tetras and rasboras.
• 1.5 litres per cm: roughly the English-speaking rule of one inch of fish per gallon, a little more cautious.
• 2 litres per cm or more: bulky or messy fish such as goldfish and many cichlids.

The rule works poorly with large fish: a 30 cm fish does not do well in 30 litres, because mass and space needs grow much faster than length.

Indicative capacity by tank

With 15% of the volume taken by substrate and decor:

An example community tank

A tank of 120 litres gross, 102 litres net, with the 1 litre per cm rule. Fish are added one group at a time:

With neons and corydoras the load is moderate; adding the otocinclus moves it into the full band. The rule says they fit, but the margin shrinks: adequate filtration and regular water changes become more important.

The reading bands

• Below 50%: a light load, with room for growth and estimation errors.
• 50% to 80%: a moderate load, a cautious band for a community tank.
• 80% to 100%: a full load, within the rule but with little margin.
• Above 100%: beyond the rule; fewer fish or a bigger tank is better.

Limits of the calculator

The centimetre-per-litre rule is a starting point, not a guarantee. It does not account for behaviour (territorial or aggressive fish), the swimming needs of some species, the minimum group size of shoaling fish, plants, filter capacity or compatibility between species. Before buying, check each species' needs in specialist sources or with an experienced retailer.

Frequently asked questions

How many fish can I keep in a 100-litre tank?

With the 1 cm per litre rule and 15% of the volume taken by substrate and decor, about 85 cm of small adult fish, for example around twenty 4 cm fish. With bulky fish, far fewer.

What is the centimetre-per-litre rule?

A rule of thumb: about one centimetre of adult fish per litre of water, for small, slender fish. It gives an order of magnitude, not a precise limit.

Should I count current size or adult size?

Adult size. Many fish are sold young and grow considerably.

Why subtract substrate and decor from the volume?

Because the stated volume is that of the empty tank: sand, rocks and wood often take up 10-20% of the space.

Does it apply to goldfish?

With caution: goldfish grow large and produce a lot of waste, so use at least 2 litres per cm or follow specific guidance.

How does it compare with the inch-per-gallon rule?

One inch per gallon is about 1.5 litres per cm, so it is a little more cautious than the European 1 litre per cm rule.

Does the filter change the capacity?

A stronger filter helps handle waste, but adds no swimming space. The calculator does not account for it.

Can I go above 100%?

Some experienced fishkeepers do with heavy filtration and water changes, but the risk of problems rises. Beginners should stay below.

Does the calculator check species compatibility?

No. Behaviour, temperature, water parameters and cohabitation must be checked for each species.