Aquarium tools · Fish & stockingUpdated October 10, 2026

Fish Stocking Calculator

Pick the fish you keep or plan to buy and see an estimate of how fully stocked the tank is. The calculator also checks the things a simple inches-per-gallon rule misses: minimum tank size, group size, shared temperature and pH, and fish that should not live together.

Your tank

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Add fish to see them in the tank. Fish are drawn at adult size.

1. Your tank

Not sure of your real water volume? Measure your tank and it will fill in here automatically.

Real water volume

US gal

Filtration

Live plants

2. Your fish

Add a fish (starts at its minimum group size)

No fish added yet.

Your list is saved on this device and shared with the compatibility and temperature checkers.

Your results

Enter your water volume and add at least one fish.

Can I add another fish?

Pick the fish you are thinking of adding

Quick answer

A common starting rule is 1 cm of adult fish per litre of water (roughly 1 inch per 4 US gallons), adjusted for body shape, filtration and plants. A 100 L planted tank holding 10 harlequin rasboras, 6 panda corydoras, 6 otocinclus and 1 dwarf gourami comes out at about 101%: fully stocked, but fine with good maintenance. Big or messy fish such as goldfish and oscars need far more room than the rule suggests, so the minimum tank size check matters more for them.

How the stocking estimate works

The calculator starts from the well-known 1 cm of fish per litre rule and improves it in three ways. Each species uses its adult length. Deep-bodied or messy fish such as angelfish, discus, goldfish and oscars carry a bioload factor above 1, because a 15 cm discus produces far more waste than five 3 cm tetras. And the tank’s capacity is nudged up for strong filtration and heavy planting.

Overstocking shows up as poor water quality: ammonia, nitrite and nitrate rise faster than the filter and your water changes can remove them, and oxygen can run short, especially at night.

Fish load = Σ (adult length cm × bioload factor × number)

Tank capacity = litres × filtration factor (0.85 / 1 / 1.15) × plant factor (1 / 1.05 / 1.1)

Stocking % = fish load ÷ tank capacity × 100

What the stocking levels mean

Stocking level, what it means and suggested weekly water change
Stocking	Level	What to expect	Weekly water change
Under 50%	Light	Very forgiving; room to add fish	20–25%
50–80%	Moderate	Comfortable for most community tanks	20–25%
80–110%	Full	Fine with a good filter and regular maintenance	25–35%
110–140%	Heavy	Needs strong filtration and frequent testing	35–50%
Over 140%	Over	High risk of ammonia, nitrite and low oxygen	35–50% and reduce fish

Tanks with demanding species such as discus get at least 30–50% per week whatever the stocking level.

Worked example: a 100 litre planted tank

Fish load: 10 harlequin rasboras × 4.5 cm = 45, 6 panda corydoras × 5 cm = 30, 6 otocinclus × 4.5 cm = 27, and 1 dwarf gourami × 9 cm = 9. All have a bioload factor of 1, so the total is 111 cm.

Capacity: 100 L × 1 (normal filter) × 1.1 (heavily planted) = 110. Stocking = 111 ÷ 110 = 101%, which is “full”. The calculator also checks that every species is above its minimum group size, that the tank meets each one’s minimum size (60 L here), and that they share a temperature range (22–25 °C) and pH range (6–7.5).

Why the percentage is not the whole story

Minimum tank size beats the rule. Two fancy goldfish only score 40% in 150 L, but OATA recommends at least 100 L for one adult plus 50 L per extra fish, so 150 L is the real minimum.
Schooling fish need a group. Neon tetras kept in threes are stressed even if the tank has room; the calculator warns below each species’ minimum group.
Shared water conditions. Discus need 28–31 °C; bronze corydoras prefer 22–26 °C. No amount of space fixes that.
Temperament and size. Adult angelfish eat neon tetras; tiger barbs nip long fins. See the compatibility checker for every pair.
New tanks need time. Add fish gradually so the filter bacteria can catch up; track the cycle first.

Frequently asked questions

How many fish can I put in a 20 gallon tank?

A 20 US gallon tank holds about 75 litres of water when full, or roughly 60–65 L after gravel and decor. By the 1 cm per litre rule that is about 60 cm of adult fish, for example 10 neon tetras (35 cm) plus 8 pygmy corydoras (20 cm). Run your exact list through the calculator to check groups and compatibility.

Is the one inch per gallon rule accurate?

It is a rough guide that works best for small, slim fish. It overestimates capacity for big, deep-bodied fish like goldfish and cichlids and ignores filtration, plants and group needs. That is why this calculator uses species-specific bioload and minimum tank sizes as well.

Can I add more fish if I have a bigger filter?

A filter rated well above your tank size adds some capacity (the calculator allows about 15%). It does not add swimming space or fix compatibility, so it cannot make a too-small tank suitable for a large fish.

Do live plants let me keep more fish?

Heavily planted tanks absorb some ammonia and nitrate, so the calculator adds up to 10% capacity. Plants are a buffer, not a replacement for filtration and water changes.

How fast can I add fish to a new tank?

Add a few fish at a time once the tank is cycled, and wait a week or two between additions while testing ammonia and nitrite. The bacteria in the filter grow to match the waste produced.

Why does the calculator say my tank is too small even though the percentage is low?

Because some fish need more swimming room or water volume than their length suggests. Minimum tank sizes come from care sheets such as OATA and Seriously Fish and override the percentage.

What to calculate next

Sources

Disclaimer: Stocking estimates are a guide based on typical adult sizes. Individual fish, filters and maintenance routines differ; test your water regularly and watch fish behaviour. See our disclaimer.

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