The volume on a fish card almost always refers not to one fish but to a whole group. That is why "I'll take two neons for a small tank" does not work: the number of litres in the catalogue already includes the shoal, and the shoal cannot be subtracted.

What the volume on a card means

Every species in the catalogue has a volume calculated for its recommended group. For neons it is "from 5–6 fish", for a pair of cichlids it is a pair. If you divide the litres by one fish, you get a wrong number, and nothing can be planned from it.

The recommended group comes from the species' keeping mode:

  • pair — 2 fish;
  • harem (a male and females) — 3;
  • small group 3–4 — 3;
  • smallish group 5–6 — 5;
  • medium group 6–8 — 6;
  • group 8–10 — 8;
  • large group 10+ — 10;
  • "shoaling" — from 5.

This number is the minimum for the species.

Why a shoal must not be cut down

The temptation is understandable: in the shop two fish are cheaper than six, and the water seems to stay the same. But shoaling fish kept alone or in pairs get stressed and fall ill. The litres per group are calculated for how the species really lives, not for how many fish are convenient to buy.

We do not advise taking fewer than the recommended group, even if the tank allows it by volume. If there is room for only one or two fish, choose a different species, not a smaller shoal. What suits a small volume is covered in the article A 10–20 litre aquarium.

If you keep more fish than the group

Each fish beyond the recommended group adds not the full volume but about 45 % of the "price of one fish". The price of one fish is the catalogue volume divided by the group size. Neighbours share the water and the filtration, so the extra fish cost less than the first ones.

A made-up example to show the arithmetic. Say a species needs 60 litres for a group of 6 fish. One fish costs 10 litres. Ten fish means four beyond the group: 4 × 10 × 0.45 = 18 litres. In total 60 + 18 = 78 litres.

The numbers here are invented for the calculation and are not taken from a card. Look up your own litres on the card of the species you want.

How different species add up

The volumes of different species are not added in full. Take the largest requirement among the species, and add half of the requirement of each next species.

Let us continue the example. A second species that needs 40 litres is added to the species that needs 78. Half is 20 litres. In total 98 litres. Simple addition would have given 118.

This arithmetic answers only the question about water. It does not say whether the fish will get along in temperament, size and conditions. For that there is the article Fish compatibility: choosing tank mates.

Litres of water and tank dimensions

Litres are counted by the water, not by the size of the glass. A tank is not filled to the brim: usually 3–5 cm is left. Substrate also displaces some of the water. So the volume "on paper" is always larger than what is actually poured in.

If all you have is the dimensions of the glass, the tank calculator has a "Calculate from dimensions" mode.

Length matters too. For long, active fish, not only the litres but also the length of the tank counts. Two tanks with the same litres may suit such a fish differently.

What does not help

  • Dividing the catalogue volume by the number of fish. The litres are given per group, so "a litre per fish" proves nothing.
  • Using a rule like "a litre per centimetre of fish". We have no such rule: the volume is taken from the species card and calculated by the steps described above.
  • Taking exactly the minimum and treating it as the goal. The volume on a card is a minimum. More water is always safer, especially for a beginner: feeding mistakes and filter failures are forgiven more easily in a large volume.
  • Adding fish gradually, "until it gets crowded". Extra fish still add litres by the rule above; you just find out later.

Choose species and see the result in the tank calculator. Put the first fish only into a tank that has been cycled — the article Starting an aquarium and the nitrogen cycle explains how. If you are just starting out, begin with the best aquarium fish for beginners.