Aquarium Stocking Level Estimator
This tool gives you a starting-point estimate of total fish length your tank can support, using the classic length-per-volume rule. Read the limitations below before you buy anything — the rule is a rough screen, not a stocking plan, and it fails badly for large-bodied, territorial, or messy species.
Estimate stocking capacity
Don’t know your real volume? Use the tank volume calculator first.
What the cm-per-litre rule actually is
The oldest stocking heuristic in the hobby is “one inch of fish per gallon of water”, which converts to roughly two-thirds of a centimetre of adult fish length per litre. It dates from an era of unfiltered or barely filtered tanks, and it survives because it is easy to remember — not because it is accurate. Metric variants (1 cm per litre, or the more cautious 1 cm per 2 litres) are the same idea with different constants.
All three variants share one core insight: volume dilutes waste, and waste production scales (very roughly) with the amount of fish tissue you keep. As a first screen — “is this idea obviously overstocked?” — the rule works. As a final answer, it does not.
Where the rule breaks down
- Body mass, not length, drives bioload. A 15 cm oscar weighs perhaps forty times as much as fifteen 1 cm ember tetras and produces waste to match. The rule treats them the same. For anything deeper-bodied than a tetra, it undercounts dramatically.
- Adult size is what counts. That 3 cm common pleco in the shop grows to 40 cm+. Always calculate with full adult size, which for many shop staples is surprisingly large.
- Territory and temperament. Two male dwarf cichlids can be “within the rule” in 60 L and still fight to the death. Footprint and sight-line breaks matter more than litres for territorial species.
- Schooling minimums. Many small species need groups of 6–10+ to behave normally. If the rule says your tank supports four rummynose tetras, the real answer is zero — keep a proper school of something smaller instead.
- Surface area and oxygen. A tall, narrow 100 L tank supports less fish than a long, shallow 100 L tank because gas exchange happens at the surface.
- Filtration and maintenance. A heavily filtered, weekly-maintained tank supports more than the same tank with a small internal filter and monthly water changes. The rule knows nothing about either.
A better workflow
- Use this tool to sanity-check the general scale of your idea.
- Research each species: adult size, group size, temperament, temperature and pH range, and swimming level.
- Cross-check the combination with a species-aware tool or an experienced keeper — forums and local clubs will happily critique a stocking list.
- Stock slowly over weeks, test ammonia/nitrite/nitrate as you go (only add more fish when ammonia and nitrite hold at zero), and judge the tank by its water quality and fish behaviour, not a formula.
Fish health note: this page is general husbandry guidance, not veterinary advice. If fish show disease symptoms (clamped fins, gasping, white spots, unusual lethargy), test your water first, and consult an aquatic veterinarian or experienced specialist for treatment decisions.
Frequently asked questions
Is the 1 inch per gallon rule wrong?
It is not so much wrong as radically incomplete. It gives sane answers for small, slim-bodied community fish (tetras, rasboras, small danios) in adequately filtered tanks, and misleading answers for nearly everything else — goldfish, cichlids, plecos, and any deep-bodied or territorial species.
Does a bigger filter let me keep more fish?
Somewhat — good filtration and generous water changes raise the ceiling, and this is exactly what the simple rule cannot see. But filtration does not create swimming space, reduce aggression, or add oxygen-exchanging surface area, so treat extra filtration as a safety margin rather than a licence to overstock.
How do goldfish fit into this?
They don’t. Goldfish are heavy-bodied, high-waste fish; common guidance is on the order of 75–100 L for the first fancy goldfish and ~40–50 L per additional fish, far beyond any length rule. Single-tailed (comet/common) goldfish ultimately need ponds.
Should I count bottom-dwellers like corydoras separately?
Count them in the total, but also check footprint: bottom-dwellers compete for floor space, not midwater volume. A tall 100 L hex tank has room for far fewer corydoras than a standard 100 L rectangle with the same volume.