Tank & Tech
Free tool

Stocking calculator

How many fish can your tank actually hold? Set the tank size and the adult size of the fish for a realistic starting range — grounded in waste load, not the one-inch-per-gallon myth.

Your tank
Adult size of the fish

Small (2–3 in): platy, guppy, cory, dwarf gourami.

Filtration and planting

Use the fish's adult size, not the size in the shop. This is a conservative starting estimate, not a hard limit.

Rough stocking estimate
6–9small fish
20 gal · standard filtration
A starting point, not a rule. Real capacity depends on the exact species, aggression and territory, how heavily you feed, and your water-change routine. Many small fish are schooling species that need groups of six or more, so plan around whole schools, and always stock slowly while a test kit confirms the tank keeps up.
Give yourself headroom
Size the filter →Monitor with a test kit →The full guide →

Why not one inch of fish per gallon?

The old rule fails because it ignores what actually limits a tank: waste, not length. A six-inch goldfish produces vastly more waste than six one-inch tetras, yet the rule treats them the same. It also ignores body shape, oxygen and swimming room. This calculator instead scales with the fish's adult size and typical bioload, which is why a big fish costs so much more of your tank than a nano one. We unpack the whole myth in how many fish can I keep.

Treat the number as a ceiling to approach slowly

The estimate is deliberately conservative, and it is a starting point rather than a target to fill on day one. Stock a tank gradually over weeks, adding a few fish at a time so the filter's bacteria can grow to match the load. Pushing to the top of the range at once spikes ammonia faster than a young filter can handle.

The things a calculator can't know

  • Aggression and territory: one cichlid may claim a tank that could hold a dozen peaceful fish.
  • Schooling minimums: many small fish need groups of six or more to feel secure, so plan around whole schools.
  • Your maintenance: frequent water changes and strong filtration buy real headroom; light maintenance shrinks it.

Give the tank headroom

Two things turn the estimate into a comfortable, stable tank: filtration and monitoring. Size the filter with the filter GPH calculator, and keep a test kit on hand to confirm ammonia and nitrite stay at zero as you add fish. If the numbers climb, you are overstocked for your current setup.

Source

  1. Estimate scales with adult fish size and typical bioload; it is a conservative planning guide, not a species-specific limit. For safety, confirm with water testing as you stock.

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