Aquarium stocking calculator

How the calculator works

Every fish adds to your tank's bioload: the waste your filter bacteria and water changes have to keep up with. The calculator estimates it from each species' adult size (not the size it is at the store) and a body-mass factor, since a deep-bodied angelfish or a goldfish makes far more waste than a slim tetra of the same length.

Your tank's capacity comes from its volume, adjusted for:

• Filtration. A filter rated well above your tank size, or a canister, handles more waste than one rated exactly for the tank.
• Live plants. Growing plants soak up ammonia and nitrate, so planted tanks get a little extra room.

The meter shows your bioload as a percentage of that capacity. Under about 85% is comfortable; 85–100% works if you stay on top of maintenance; over 100% means you need more filtration, more water changes, or fewer fish.

It checks more than bioload

A tank can be under 100% and still be a bad mix. The calculator also warns you about:

• Minimum tank size. Active or large fish like Denison barbs, rainbowfish and clown loaches need swimming length, not just water volume.
• Group size. Tetras, rasboras, corys and otos are stressed and hide in small numbers. The first time you add a schooling fish, it adds a full group.
• Temperature. It finds the range every fish you picked can share, and flags mixes with no overlap, like goldfish and discus.
• Compatibility. Fin nippers with long-finned fish, predators with bite-sized fish, cichlids and bettas with shrimp, and species that fight their own kind.

Why "one inch per gallon" falls short

The classic rule says one inch of fish per gallon of water. It's a decent starting point for small, slim fish, but it breaks down fast:

Waste scales with body mass, which grows much faster than length. That's why this calculator weights every species individually.

Tips for stocking a new tank

• Cycle first. Let beneficial bacteria establish before adding fish, and test for ammonia and nitrite.
• Add fish slowly. A few at a time over several weeks lets the bacteria catch up with each new load.
• Plan for adult size. Plecos, angelfish and rainbowfish sold at 1–2 inches grow to 4–6 inches or more.
• Trust your test kit. Any stocking rule is an estimate. Rising nitrates between water changes mean you're closer to the limit than the math says.

FAQ

Do live plants let me keep more fish?

A little. Growing plants take up ammonia and nitrate, so the calculator gives planted tanks about 10% more room. They don't replace water changes or filtration.

What does overstocked mean?

The fish produce more waste than the tank and filter can comfortably process between water changes. Signs include ammonia or nitrite readings, nitrates climbing fast, cloudy water, and stressed or aggressive fish.

Do shrimp and snails count?

Yes, but barely. Neocaridina shrimp add very little bioload, which is why a 10 gallon can hold a big colony. Snails like mystery snails add more than you'd expect.

Can I add all my fish at once?

No. Even in a fully cycled tank, add fish a few at a time and test the water after each addition.