Stocking
Estimate how many fish your tank can hold. A size-weighted bioload estimate for any aquarium size and species mix, with minimum tank size warnings.
Estimate how many fish your tank can hold. Set your tank size, add species, and watch the stocking level: a size-weighted estimate of bioload, not the rough “one inch per gallon” rule.
Room for roughly 3 more neon-sized fish (about 1.5 in).
This is an estimate, not a safe-capacity guarantee. It weights each species’ typical adult length by body size; filtration, plants, maintenance, and individual fish all move the real limit. Let nitrate tests and your fish’s behavior have the final say.
Next step
Found your mix? Find an aquarium store near you that carries these species and the gear to support them.
Find stores near meQuick answer
Weigh each species' adult size against your tank's net water volume instead of the rough one-inch-per-gallon rule. As an estimate, a 10-gallon holds about 6-8 neon-sized fish (about 1.5 in) or 4-5 two-inch fish like guppies; a 29-gallon holds a small community plus a centerpiece fish.
Quick reference
How to use
Stocking is about bioload, not a length-per-gallon rule. This aquarium stocking calculator estimates how heavily stocked a tank would be by weighting each species' adult length by body size and comparing the total with your tank's net water volume. Use it to sanity-check how many fish fit in a 10, 20, 29, or 55 gallon tank; it's a heuristic estimate, not a guarantee.
Enter your tank's rated size in gallons. The estimate already takes off about 10% for substrate and decorations, so don't subtract it yourself. If you only know the dimensions, run the tank volume calculator first.
Pick each species and quantity. The calculator uses each fish's typical adult length (not the size in the store tank today), counts bigger fish more per inch as a stand-in for bioload, and warns if a species needs a bigger tank.
The result shows an estimated stocking percentage: under 70% leaves room to spare, 70-95% is well stocked, 95-110% is at capacity, and over 110% is overstocked. Treat it as an estimate and let nitrate tests have the final say.
Add fish a few at a time over several weeks so your biological filter can keep pace. A fully stocked new tank crashes; a slowly stocked one stabilizes.
Common sizes
Pro tips
Quick facts
Pitfalls
Learn from others' errors to ensure accurate calculations and healthy aquatic life.
Why it matters — It ignores body mass, so it lets you overstock with big, bulky fish (and can be needlessly strict with tiny ones)
Do this instead — Use a size-weighted estimate as a starting point, respect each species' minimum tank size, and confirm with nitrate tests
Why it matters — Ammonia spikes the young filter can't process, killing fish
Do this instead — Add fish gradually over several weeks so beneficial bacteria can keep up
Why it matters — A cute 1-inch fish becomes a 12-inch tank-buster
Do this instead — Always plan stocking around full adult size, not the size in the store
Why it matters — Stressed, nippy, hiding fish when kept in groups that are too small
Do this instead — Keep schooling species in groups of 6+ and count every one toward your stocking total
Source locally
To stock your tank, you'll want:
Products
tropical fish · aquarium filters · test kits · water conditioners
Services
fish ordering · stocking consultation
Reference
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