Aquarium Stocking Calculator

Add the fish you keep or plan to keep. You get a stocking level for your tank and filter, and warnings where species need a bigger tank, a larger group or different water.

Units
The volume I know is
We then assume a normal filter that moves the tank volume 5 times an hour.
The GPH or L/h printed on the filter box.
118 species. Adding a schooling fish adds its smallest group.
Stocking level67%Well stocked
Animals173 species

17 animals in about 34 US gal (129 L) of water (85% of the label size) is 67% of what this tank and filter can carry: well stocked. There is room to spare, which keeps the water stable. The figure assumes adult size, a cycled filter and a weekly water change.

Bioload
28.3 points of 42.5 the tank can carry
Water used for the sums
34 US gal (129 L), estimated from the label size
Filter turnover
5.9× the tank volume an hour
Longest fish this tank suits
about 9 in (23 cm) at adult size: a quarter of the tank length
Temperature that suits all
72–79 °F (22–26 °C)
pH that suits all
6–7.5
0%95%110%
  • Neon tetraParacheirodon innesi · 3.5 cm (1.4 in) adult · 11 pts
  • Bronze corydorasCorydoras aeneus · 6.5 cm (2.6 in) adult · 15.4 pts
  • Honey gouramiTrichogaster chuna · 5 cm (2 in) adult · 2 pts
  • No conflicts found between these species.

What to work out next

Filter flowIs the filter strong enough?Water changeRoutine for this stock levelHeater sizeWatts to hold their temperature

How the stocking level is worked out

Every animal gets a number of bioload points based on its adult size and build, and the tank gets a capacity based on its real water volume and filter. The stocking level is one divided by the other.

Stocking level = total bioload points ÷ tank capacity × 100One point is roughly the waste that one US gallon (3.8 L) of tank size can carry.

Bioload points per fish. For fish under 3 inches (7.6 cm) we use the classic rule of one inch of fish per gallon of tank size. Above that size the rule breaks down, because a fish's body mass, and so the food it eats and the waste it makes, grows much faster than its length. So from 3 inches up, the points rise with the square of the length:

Points = length × (length ÷ 3) × buildLength in inches at adult size. Build is 0.8 for slim fish (tetras, rasboras), 1 for average fish, and 1.5 for heavy-bodied, messy fish (goldfish, oscars, plecos).

A 1.5-inch neon tetra scores about 1.2 points. A 12-inch oscar scores about 70: as much as 60 neon tetras, which matches what keepers of large cichlids find in practice.

Tank capacity. Capacity starts at 1.25 points per US gallon of real water. That equals the traditional one point per gallon of label size, because a set-up tank holds about 80% of its label volume. It is then adjusted for filtration: a filter turning the tank over fewer than 4 times an hour lowers it by 10–20%; 6 or more turnovers raise it by 10–15%. A heavily planted tank adds 10%, since plants take up ammonia and nitrate directly.

Stocking level Meaning
Up to 60% Lightly stocked: very stable, forgiving of missed water changes
60–95% Well stocked: the range to aim for
95–110% Fully stocked: works with strong filtration and weekly 30–50% water changes
Over 110% Overstocked: water quality will slide between changes

What the warnings check

Volume is only half of stocking. The calculator also checks each species against the tank and against each other:

  • Minimum tank size for the adult fish, however few you keep.
  • Tank length: a tank should be at least four times the length of the adult fish.
  • Group size: schooling and shoaling fish kept in too small a group become stressed, shy or nippy.
  • Temperament: aggressive fish with peaceful ones, and fin-nippers with long-finned fish.
  • Predation: fish that grow big enough to swallow their tankmates.
  • Temperature and pH: whether one range suits everything on the list.

The limits of any stocking calculator

A stocking figure is a planning aid, not a guarantee. It assumes a filter that has finished the nitrogen cycle, adult fish, sensible feeding and a weekly water change. Real tanks vary: individual fish have their own temper, and a tank can be within its bioload yet too cramped for a territorial pair. Add fish a few at a time over several weeks, test the water, and let the fish's behavior have the last word.

Common mistakes

  • Stocking for the size of fish in the shop. Most are sold as juveniles: plan for the adult size.
  • Adding all the fish at once. The filter bacteria need time to grow: add a few fish every one to two weeks.
  • Keeping schooling fish in twos and threes. Six is the usual minimum, and more is better.
  • Trusting "one inch per gallon" for big fish. A 10-inch fish is not equal to ten 1-inch fish: it produces many times the waste.
  • Using the label volume. Use the real water volume from the volume calculator.

Questions and answers

How many fish can I put in my tank?

For small fish under 3 inches, about one inch of adult fish per US gallon of tank size (roughly 1 cm per 1.5 liters) is a safe guide. Larger fish need far more: use the calculator, which scales the load with body size and checks the minimum tank for each species.

Is the one inch per gallon rule accurate?

Only for small, slim fish. It badly underestimates large or heavy-bodied fish such as goldfish, oscars and plecos, and it says nothing about swimming room, aggression or group size. This calculator keeps the rule for small fish and corrects it for big ones.

Does a bigger filter let me keep more fish?

A little. Stronger filtration processes waste faster and adds oxygen, but it does not add swimming room or dilute nitrate, which only water changes remove. The calculator allows up to 15% extra for strong filtration.

Is this an AqAdvisor alternative?

Yes. It answers the same question, using a method we publish in full on this page, works on phones, and shows results in both gallons and liters.

Look up a fish

Tank size, group size and tank mates for the most popular species.

Related guides

Sources

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