How many fish can I put in my tank?

There's no single number — it depends on each species' bioload, your filtration, and how much surface area the tank has. This calculator adds up the bioload of your planned fish and compares it to a capacity based on your net water volume and footprint, then flags when you're near or over the limit. Aim to stay at or below 100% and keep filtration and water changes in step.

Why is the inch-per-gallon rule unreliable?

It assumes every inch of fish creates the same waste, but body shape and metabolism vary enormously — a 10-inch pleco pollutes far more than ten 1-inch tetras. It also ignores aggression, schooling needs, and tank shape. A bioload model with per-species factors is far closer to reality.

Does tank shape really matter?

Yes. Two tanks can hold the same volume but a long, shallow one has more water surface for gas exchange and more floor length for swimming, so it supports more fish than a tall, narrow column. The calculator applies a footprint factor for this and warns when a tank is too short for active swimmers.

What does the stocking percentage mean?

It's your total bioload as a share of the tank's estimated capacity. Under 50% is lightly stocked, around 85–100% is well stocked, and over 110% is overstocked. It's a planning guide, not a hard limit — strong filtration, live plants, and diligent water changes give you some headroom.

Can I save my tank setup?

Yes. Name your setup and save it to your browser with the profile tool; it stores your tank dimensions, filter, and species list locally so you can reload and tweak it on your next visit. Nothing is uploaded.

Free aquarium calculator

Aquarium Stocking Calculator

Plan a healthy community. Set up your tank, add species, and get a bioload-based stocking level, filtration capacity, a water-change schedule, and compatibility warnings that explain why — not just a red flag.

Your tank

Dimensions set the net water volume and footprint everything is anchored to.

Tank shape

Dimension units

Standard tank sizes

Dimensions

Filter flow rate

On your filter's box or specs. Leave blank to skip the filtration check.

Substrate & décor (net volume)

Subtract what displaces water for a realistic fill — the gap most calculators ignore.

Substrate depth

Décor displacement

Livestock

Search species

Or browse by group

No species yet. Add one above to start stocking.

Stocking summary

— Add species to begin

Filtration capacity: Enter your filter's flow rate to check filtration capacity.
Water change: —

Compatibility & care notes

No conflicts found for this combination. Still research each fish before you buy — temperament varies.

Stocking results are estimates, not guarantees. Real compatibility depends on individual temperament, tank layout, and your maintenance. Research each species and watch how your fish actually behave.

Tank profiles

Save this setup in your browser and reload it on your next visit.

No saved profiles yet.

How the stocking calculator works

Set up your tank, add the fish you're planning, and the calculator returns a stocking level, the filtration your bioload needs, a water-change schedule, and plain-English compatibility notes. It's anchored to your tank's net water volume and footprint — not the empty rated size — so the numbers reflect the water your fish actually live in.

Bioload, not inches per gallon

The old inch-per-gallon rule treats a slim tetra and a heavy-bodied goldfish as equal, which they aren't. Instead, every species carries a bioload factor — its relative waste and oxygen demand, with a neon tetra as the 1.0 baseline. The tank's capacity scales with its net volume and a footprint factor that rewards surface area, because a long, shallow tank exchanges more gas and offers more swim room than a tall column of the same volume. Stocking level is total bioload divided by that capacity.

Worked example

A 75-litre community over a 75 × 30 cm footprint has a capacity of about 19 bioload units. Add 8 neon tetras (8 units) and 6 corydoras (9 units) and you're at roughly 90% — well stocked but not crowded. The model suggests roughly 7–8× turnover (about 570 L/h of filtration) and a 35% weekly water change. Swap in a common pleco and the tank is instantly overstocked and too small.

Filtration & water changes

Recommended filter flow ramps from about 4× the tank volume per hour when lightly stocked up to 8× when full, so heavier bioloads need stronger filtration. Enter your filter's rated flow to see how it compares. The water-change schedule scales the same way — a lightly stocked tank stays clean with a small weekly change, while a near-capacity tank needs a larger one.

Frequently asked questions

How many fish can I put in my tank?: There's no single number — it depends on each species' bioload, your filtration, and how much surface area the tank has. This calculator adds up the bioload of your planned fish and compares it to a capacity based on your net water volume and footprint, then flags when you're near or over the limit. Aim to stay at or below 100% and keep filtration and water changes in step.
Why is the inch-per-gallon rule unreliable?: It assumes every inch of fish creates the same waste, but body shape and metabolism vary enormously — a 10-inch pleco pollutes far more than ten 1-inch tetras. It also ignores aggression, schooling needs, and tank shape. A bioload model with per-species factors is far closer to reality.
Does tank shape really matter?: Yes. Two tanks can hold the same volume but a long, shallow one has more water surface for gas exchange and more floor length for swimming, so it supports more fish than a tall, narrow column. The calculator applies a footprint factor for this and warns when a tank is too short for active swimmers.
What does the stocking percentage mean?: It's your total bioload as a share of the tank's estimated capacity. Under 50% is lightly stocked, around 85–100% is well stocked, and over 110% is overstocked. It's a planning guide, not a hard limit — strong filtration, live plants, and diligent water changes give you some headroom.
Can I save my tank setup?: Yes. Name your setup and save it to your browser with the profile tool; it stores your tank dimensions, filter, and species list locally so you can reload and tweak it on your next visit. Nothing is uploaded.