Tool
Stocking level calculator
A very rough check on how many fish a tank can carry — and why the honest answer is "it depends".
Your result
Roughly 15 adult fish of 4 cm
This is a rough starting estimate only — the biological load, filtration and water-change routine matter far more than a number.
What this calculator does
Stocking is one of the most argued-about topics in fishkeeping, and the old "one inch per gallon" rule is widely considered too crude. This tool uses a volume-based guide that scales with fish size and body mass, then converts it into a rough number of adults. Treat the answer as a starting point to sanity-check a plan, never as a target to fill up to.
When to use it
- Deciding whether a tank is big enough for a species.
- Checking a shopping list before buying.
- Understanding why bigger fish need disproportionately more room.
How to use it
- Use the real water volume from the volume calculator, not the nominal tank size.
- Pick the closest fish type — messier or larger fish need more water each.
- Enter the adult size, not the size the fish is now; small juveniles grow.
Worked example
A 100-litre tank of small community fish, planned around fish that reach 4 cm, works out at roughly 15 adults. Fill it with the same stocking of 10 cm fish and the honest figure drops to about 6 — because a larger fish uses far more than twice the water of a smaller one.
What the number leaves out
- Filtration. Good biological filtration lets a mature tank carry more than a new one.
- Water changes. Frequent, generous changes support more fish than a lazy routine.
- Swimming space and territory. Some fish need length or a cave, not just litres.
Limitations
- It is deliberately rough. Two tanks of the same size can support very different loads.
- Behaviour is not counted. Territorial or nippy fish need more space than their size suggests.
- Filter by fish, not numbers. Check each species' adult size and requirements rather than aiming to "fill" a tank.
Frequently asked questions
Is "one inch per gallon" still used?
It is outdated and misleading because it ignores body mass, habits and waste. Most keepers now use volume, filtration and water-change frequency together.
How many fish can a 100 L tank hold?
There is no single answer. For small community fish it may be plentiful; for a large cichlid it can be one or none. Match the tank to the adult fish.
Does a bigger filter mean I can add more fish?
Better filtration helps, but it does not change the space each fish needs or the waste they produce. Stock to the tank, not to the filter.
Related guides
Sources and research notes
Stocking rules vary widely between sources and should be treated as guides, not rules; the underlying principle is that larger fish need far more water per fish.