Skip to main content
Redmoon Converters
EN
English en Español es Português (Brasil) pt-BR Français fr Deutsch de Italiano it Nederlands nl 日本語 ja 简体中文 zh-Hans 한국어 ko العربية ar עברית he اردو ur हिन्दी hi मराठी mr తెలుగు te বাংলা bn ਪੰਜਾਬੀ pa
🐾 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Get tank volume from L × W × H and realistic stocking guidance using adult fish size and bioload, not the outdated "1 inch per gallon" rule. Filter and temperature compatibility checks.

Tank capacity
— L · — US gal
Stocking: — · Filter turnover: —
SpeciesQtyAdult sizeBioload
How it works

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 The outdated "1 inch per gallon" rule ignores body shape — a 30 cm goldfish produces 50× the waste of a 5 cm tetra.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.

Also known as

aquarium stocking calculatorfish tank stockinghow many fish in my tank

Related tools