Skip to main content
Redmoon Converters
🐾 Pets & Animals

Aquarium Fish Stocking Calculator

Get tank volume from L × W × H and realistic stocking guidance using adult fish size and bioload, not the outdated "1 inch per gallon" rule. Includes a filter turnover check and schooling-minimum warnings.

Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on · Corrections log

Tank capacity
— L · — US gal
Stocking: — · Filter turnover: —
SpeciesQtyAdult sizeBioload
Quick reference

Volume = L × W × H (in cm) / 1000 = litres. We subtract 10% for substrate, rocks and equipment displacement. The outdated "1 inch per gallon" rule ignores body shape — a 30 cm common goldfish is rated here at 30× the bioload of a 3 cm neon tetra.

Bioload sums species-specific waste ratings (low / medium / high / very high). Filter turnover should be at least 4× tank volume per hour for community tanks, 6–10× for heavily stocked or messy fish like goldfish and cichlids.

How it works

Volume is length × width × height converted to centimetres, divided by 1000 for litres, then cut by 10 percent for substrate, rock and equipment displacement — the default 90 × 30 × 35 cm tank reports 85.1 L rather than 94.5 L. Stocking then ignores inches per gallon entirely. Each species carries its own bioload number, from Neon Tetra at 1 through Angelfish at 7 to common Goldfish at 30, and tank capacity is litres divided by four.

Species selection dominates the result. One common goldfish at 30 points consumes the same capacity as thirty neon tetras, which is precisely the argument against length-based rules. Filter GPH drives the second output: turnover is GPH × 3.785 divided by litres, and anything under 4× per hour raises an under-spec warning that prints the GPH you should actually be shopping for. Adding fewer fish than a species' listed school size triggers its own separate warning.

The list is seventeen freshwater species and nothing more — no marine fish, no invertebrates, no plants, and no way to enter a species that is not there. Despite what the summary suggests, there is no temperature, pH or aggression check anywhere in the calculation, so it will let you stock Discus alongside common Goldfish without complaint. The bioload figures also assume a fully cycled, well-maintained tank and model nothing about water-change frequency.

Frequently asked questions

How many litres is a tank measuring 90 × 30 × 35 cm?

Raw volume is 94.5 litres, but the tool reports 85.1 because it subtracts ten percent for substrate, rock and equipment displacement. That lower figure is what the stocking percentage is measured against, and it is the honest number to plan your stocking around.

Why is the one inch per gallon rule wrong?

Because waste output does not scale with body length. In this tool a common goldfish carries 30 bioload points against a neon tetra's 1, so a single goldfish occupies as much capacity as thirty tetras despite being only ten times as long.

What filter GPH do I need for my tank?

Turnover is your filter's GPH multiplied by 3.785 and divided by tank litres. Below four turnovers an hour the tool warns and prints the GPH that would reach 4×. Heavily stocked tanks and messy fish want considerably more than that baseline.

Why am I being warned about schooling fish?

Each species carries a minimum group size — six for neon tetras, cardinals, zebra danios and corydoras, five for discus, three for guppies, platies and mollies. Adding fewer than that triggers the warning, because under-schooled fish are stressed and tend to colour down.

Can I check whether two species will get along?

Not with this tool. It handles volume, bioload and filter turnover only, with no temperature, pH or aggression matching at all, so it will happily let you stock discus with goldfish. Check compatibility from a separate source before buying anything.

Why the inch-per-gallon rule is a poor guide

The familiar rule of thumb treats fish length as the variable that matters, and it fails in both directions. Bioload scales closer to body mass than to length, so a single fish twice as long produces far more waste than two of half the size. And it ignores adult size entirely — the rule is routinely applied to juveniles that will triple in length.

Territory and behaviour are not in it at all. A tank correctly stocked by volume can still be badly overstocked for a territorial species that needs floor area, or understocked for a schooling species that needs numbers to behave normally.

Filtration, cycling and treating the figure as a ceiling

Stocking capacity is really a question about biological filtration — whether the nitrifying bacteria colony can process ammonia as fast as the fish produce it. That colony takes weeks to establish, which is why a tank must be cycled before stocking and why adding fish gradually matters more than the final number.

Filter turnover, surface area for gas exchange, planting and water-change frequency all shift real capacity substantially. Treat any stocking figure as a ceiling to stay under rather than a target to fill, and research the adult size and temperament of each species individually.

Related tools