Fish Stocking Calculator — How Many Fish Fit Your Tank

← Research ReefThe Creative Island · Island 13

The eternal question — how many fish? Volume, fish size, and a rule of thumb with its limits stated honestly.

Tank volume (litres, actual water)

Adult size of fish (cm)

Stocking approach Conservative (1 cm fish per 2 L) Standard (1 cm fish per 1 L) Heavy (1 cm per 0.5 L — experts, heavy filtration)

Rule-of-thumb stocking is a starting point, not a law — research your species' adult size, shoaling needs, territory and bioload before buying.

How to use this tool

1. Enter your tank's actual water volume in litres (use the Aquarium Volume Calculator for the honest figure).
2. Enter the fish's adult size in cm — not the size in the shop tank.
3. Choose a stocking approach and press Calculate stocking.

Example

A 56 L tank with 5 cm adult tetras, standard rule: 56 ÷ 5 ≈ 11 fish — a proper shoal. The same tank with 12 cm goldfish: 4 fish by arithmetic, but honestly fewer, because goldfish are messy bioload machines.

The formula

fish count ≈ volume ÷ (adult cm × litres-per-cm factor)

What the result means

The litres-per-fish rule encodes a biological truth: water volume and filtration set how much waste bacteria can convert. It ignores species behavior — territory, shoaling, aggression — which is why the result is a ceiling to think under, not a target to hit. Adult size matters most: the 3 cm shop juvenile that grows to 15 cm turns a stocked tank into an overcrowded one.

Frequently asked questions

Is '1 cm per litre' actually reliable?

It's a useful sanity ceiling for small peaceful community fish, and wrong for big, messy or territorial species. Use it to reject bad ideas ('20 goldfish in 40L'), not to design a biotope.

Should I stock to the maximum?

No — experienced keepers stock 50–70% of the ceiling. Understocked tanks are stable, forgiving and easier to keep beautiful.

Does it work for bettas and other solitary fish?

Volume-wise yes, but solitary/territorial species override the count: one betta regardless of what the arithmetic allows, and territorial cichlids need footroom, not just litres.

Is my aquarium data uploaded?

No — the estimate is computed in your browser.

More tools from Research Reef

Aquarium Volume CalculatorTank litres & gallonsDilution CalculatorC1V1 = C2V2Probability CalculatorOdds, fractions, diceScientific Calculatorsin, cos, log, powers

Explore the island world

• Research Reef — the island where this tool lives.
• The Main Island — drive the boat, explore the whole world.
• The 90s Kids Island — where most of us started.

Privacy & data: this tool runs entirely in your browser — nothing you type is uploaded, saved or sent anywhere. · Disclaimer: calculations use standard scientific formulas — for lab, safety or field decisions, always double-check with official protocols. · Privacy Policy · Disclaimer