What is an aquarium stocking calculator?
An aquarium stocking calculator estimates how many fish, shrimp and snails a tank can hold safely. It compares the waste your animals produce (their bioload) with what your tank volume, filter, plants and water changes can handle, then flags species that should not share a tank. Use it to plan a new tank, check a purchase before you buy, or find out whether your current tank is overstocked.
How this fish tank stocking calculator works
Bioload from adult size
Every species has an adult size, and larger fish produce much more waste than their length suggests. The calculator raises each fish's adult length to a power greater than one, multiplies by the quantity, and adds everything up. This is why 12 small tetras and one oscar are not treated as roughly equal.
Tank capacity
Capacity starts from your tank volume in US gallons, then adjusts for filtration strength, live plants and how often you change water. The stocked percentage is total bioload divided by capacity. Stronger filtration, plants and weekly water changes raise capacity a little, and weaker maintenance lowers it.
Compatibility checks
The calculator also warns you when a species needs a bigger tank than yours, when a schooling fish is kept below its minimum group size, when temperature ranges do not overlap, when aggressive species share a tank with peaceful ones, and when fin nippers share a tank with long-finned fish.
What do stocking percentages mean?
| Stocked | Status | What to expect |
|---|---|---|
| 0–50% | Lightly stocked | Easy to maintain and forgiving for beginners, with stable water. |
| 50–80% | Moderately stocked | A balanced tank that needs regular water changes and weekly testing. |
| 80–100% | Fully stocked | Needs strong filtration and diligent maintenance. Not ideal for beginners. |
| Over 100% | Overstocked | High risk of ammonia spikes, stress, aggression and disease. Reduce the stock. |
Beginners do best staying below about 80%. Experienced keepers with strong filtration and a good water-change routine can run a tank fuller.
How many fish in a tank? Example stocking by tank size
These are starting points you can load into the calculator and adjust. Species choices, not just tank size, decide what works.
| Tank size | Example stock | Notes |
|---|---|---|
| 5 gallons | 1 betta, or a small group of chili rasboras with shrimp | Keep only one betta. Nano fish suit this size. |
| 10 gallons | 8 ember tetras, 6 cherry shrimp, 1 nerite snail | A peaceful nano community. |
| 20 gallons | 1 honey gourami, 6 harlequin rasboras, 3 amano shrimp | A calm community with one centerpiece fish. |
| 29 gallons | 10 neon tetras, 6 panda corydoras, 2 otocinclus | Cory catfish need a soft substrate and a group. |
| 55 gallons | 12 harlequin rasboras, 8 cherry barbs, 6 bronze corydoras, 1 pearl gourami | Good room for several schools at different swim levels. |
Why the inch-per-gallon rule falls short
The old rule of one inch of fish per gallon of water is easy to remember, but it ignores several things that matter more than total length:
- Body mass: a 4-inch goldfish produces far more waste than four 1-inch tetras.
- Adult size: fish at the store are usually juveniles.
- Behavior: territorial and active fish need space, not just water volume.
- Filtration and plants: two tanks of the same size can support different loads.
How to stock a new aquarium step by step
- Measure your real water volume. Substrate and decor take up space, so use the volume the tank holds, not the label on the box.
- Cycle the tank before adding fish. Fish waste becomes ammonia, which is toxic. Beneficial bacteria turn it into nitrite and then nitrate, a process called the nitrogen cycle (UF/IFAS Extension). RSPCA Australia suggests testing for ammonia and nitrite every two to three days for the first six weeks of a new tank (RSPCA Australia).
- Pick compatible species. Match temperature, temperament and swim levels. Look up each species' adult size and habitat in a reliable species database before you buy.
- Add fish in small groups. Add a few at a time and test the water between additions so your filter bacteria can keep up.
- Maintain the tank. Routine partial water changes, such as about 10% a week in RSPCA Australia's guidance, keep nitrate in a safe range.
Common aquarium stocking mistakes
- Buying fish by their size in the shop instead of their adult size.
- Adding the whole stock on the same day to a new tank.
- Keeping schooling fish alone or in pairs, which causes stress and nipping.
- Mixing species that need different temperatures.
- Forgetting that shrimp and snails add to bioload too.
- Skipping a water test kit and relying only on a calculator.
How accurate is this calculator?
It gives a reasoned estimate from species size, tank volume and care habits, and like any stocking estimate it is guidance, not a guarantee. It cannot predict an individual fish's personality, your exact water chemistry, or how closely you follow your maintenance plan. Treat the result as a planning aid, research each species, and keep a backup plan for rehoming fish that do not get along.
Frequently asked questions
It depends on the adult size of each fish, how much waste it produces (its bioload), your filtration, live plants and how often you change water. Use the calculator above and aim to stay under about 80% stocked, especially in a new tank.
Only as a rough starting point. It ignores body shape, adult size, activity and behavior, so a slim neon tetra and a bulky goldfish of the same length are treated alike. This calculator weighs adult size more heavily and adds compatibility checks.
Bioload is the total waste load that fish, invertebrates and uneaten food put on your tank. Waste breaks down into ammonia, which is toxic to fish, so a tank can only hold as many animals as its filter bacteria and water changes can handle.
Common signs are ammonia or nitrite readings above zero in an established tank, nitrate that climbs back up quickly after water changes, fish gasping at the surface, constant aggression and cloudy water. A water test kit is the most reliable check.
Add a few at a time rather than the full stock at once. New fish add waste, and filter bacteria need time to catch up. Test for ammonia and nitrite between additions.
Yes, modestly. Stronger filtration and live plants help process waste, and regular water changes keep nitrate down. They do not remove the need for enough space, swimming room and compatible species.
No. It covers freshwater fish, shrimp and snails only. Marine and reef tanks have different bioload and compatibility rules.
Usually not. Their temperature ranges differ, so one group ends up too warm or too cold. The calculator warns you when the species you add have no shared temperature range.
For a rectangular tank, multiply length by width by height in inches and divide by 231 to get US gallons. In centimeters, multiply the three and divide by 1,000 to get liters. Subtract about 10% for substrate and decor.
Last updated: . This tool is for planning only and is not veterinary advice. See our Disclaimer.