What this guide calculates — and why it should not be trusted alone
This guide applies the traditional centimetres-of-fish-per-litre rule, with a safety reduction.
That rule is widely regarded as unreliable and is no longer recommended by most fishkeeping authorities. It is here because it remains in circulation and because understanding why it fails is more useful than a number. The problems are set out below.
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Why the rule fails
- Fish are three-dimensional. A 10 cm fish is not twice the mass or waste output of a 5 cm one — it is closer to eight times.
- It ignores species entirely. A goldfish and a neon tetra of the same length produce vastly different waste loads and need vastly different space.
- It ignores behaviour. Shoaling species need groups; territorial species need footprint; active swimmers need length.
- It ignores filtration, planting and maintenance, all of which change what a tank can support far more than volume alone.
Gas exchange happens at the water surface, so a wide shallow tank supports more fish than a tall narrow one of the same volume. Territorial and bottom-dwelling species care about floor area, not litres. Two tanks holding the same water can be suitable for completely different stockings.
How this calculator works
The traditional rule, with a reduction:
Centimetres of fish = tank volume × allowance per litre × (1 − safety reduction ÷ 100)
Commonly quoted allowances are around 1 cm per litre, sometimes lessThe safety reduction exists because the rule is optimistic even on its own terms. Applying 20% or more is sensible, and applying the rule at all is only reasonable for small, similar, well-understood community species in a mature filtered tank.
The reliable approach is researching the specific species: adult size, minimum group size, minimum tank footprint, temperament and bioload. Reputable species profiles give all five.
Worked example: a 60 litre tank
Using the default figures — 60 litres at 1 cm per litre with a 20% reduction:
- Result: 48 cm of fish
Read literally, that permits one 48 cm fish, or forty-eight 1 cm fish, or four 12 cm goldfish. All three are absurd. A single common goldfish reaches 20 cm or more and needs well over 100 litres; forty-eight fish in 60 litres would overwhelm any filter; and a 48 cm fish would not turn round. The number is arithmetically correct and biologically meaningless, which is the clearest possible demonstration of why the rule was abandoned.
Common mistakes
- Using the rule as a stocking decision. It ignores everything that actually determines capacity.
- Stocking to adult size on the size in the shop. Many species sold small grow substantially.
- Adding all the fish at once. A new filter cannot process the waste; stock gradually over weeks.
- Keeping shoaling species in twos and threes. Group size is a welfare requirement, not a preference.
- Ignoring footprint for bottom-dwellers. Floor area matters more to them than volume.
Frequently asked questions
What should I use instead?
Species research. For each fish: its adult size, the minimum tank dimensions recommended for it, its minimum group size, its temperature and water parameter needs, and its compatibility with the others. Reputable aquarium societies and species databases give all of these, and the answer is frequently that fewer species in larger groups works better than more species in small ones.
What is cycling a tank?
Establishing the bacterial colonies in the filter that convert fish waste — ammonia to nitrite to the far less harmful nitrate. It takes several weeks and must happen before fish are added, or before more than a very few are. Adding a full stocking to an uncycled tank exposes the fish to ammonia and nitrite, which is a common cause of early losses.
Do fish grow to the size of the tank?
No. This is a persistent and harmful myth. Fish in a tank too small for them suffer stunted growth, deformed skeletons, suppressed immunity and shortened lives — the body is constrained while the internal organs continue developing. A common goldfish reaches 20 cm or more given the space it needs, and does not stop growing because the tank is small.
How often should I change the water?
Typically 20 to 30% weekly for a normally stocked tank, more for a heavily stocked one. Regular partial changes remove nitrate and replenish minerals, and they are the single most effective maintenance task. Testing rather than guessing tells you whether the schedule is adequate.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
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References
- GOV.UK — Animal Welfare Act duties towards kept animals, including fish
- RSPCA — welfare guidance on keeping fish, tank size and stocking
Sources are checked at publication and can change — how I choose and check references.