Aquarium Stocking Calculator
Tell us your tank size and the fish you're thinking about. We'll work out how full your tank really is — using each fish's adult size and bioload, not the misleading inch-per-gallon rule — and flag problems before your fish have to live with them.
Better filtration buys a little headroom — it never makes a too-small tank suitable for a big fish.
How this calculator works
Every fish has a stocking load: its adult length multiplied by a bioload factor. Slim, light-waste fish like tetras score below 1 (0.6); heavy-bodied waste producers score above it — a fancy goldfish scores 2.5, because your filter experiences an 8-inch goldfish as far more than "8 inches of fish." Add up the load of everything in the tank, divide by the tank's capacity (gallons, adjusted slightly for filtration), and you get a percentage.
The bands are deliberately conservative:
- Up to 70% — lightly stocked. Very forgiving. Ideal for a first tank.
- 70–90% — comfortably stocked. A full, lively tank that still forgives a late water change.
- 90–110% — fully stocked. Workable, but maintenance must be consistent and there is no room to add "just one more."
- Over 110% — overstocked. The most common setup behind ammonia spikes and unexplained fish deaths.
The calculator also checks the things a percentage can't see: whether each species' minimum tank size is met, whether schooling fish have a proper group, whether all species share a common temperature, and known bad combinations (a growing angelfish with neon tetras, a betta with a gourami, goldfish with tropical fish).
What this calculator can't do
No calculator replaces judgement. It doesn't know your tank's footprint (long tanks suit active swimmers better than tall ones of the same volume), your aquascape, or individual fish temperaments, which vary. Treat a passing result as "a sensible plan," not a guarantee — and always cycle the tank fully before any fish goes in.
Common questions
Why not just use 1 inch of fish per gallon?
Because it treats a 12-inch common goldfish (a pond fish that produces enormous waste) as equal to twelve 1-inch tetras. We explain the rule's failures in detail here.
Can I keep a goldfish in a 10-gallon tank?
No. Even a fancy goldfish reaches about 8 inches and produces heavy waste; it needs at least 20 gallons for the first fish. A common goldfish grows to a foot and belongs in 55+ gallons or a pond. Try one of our 10-gallon plans instead — there are much better fits for a small tank.
My result is just over 100%. Is that an emergency?
Not an emergency, but a warning: keep up weekly water changes of 35–40%, don't add anything else, and plan around the fish growing to full adult size. If fish are still small juveniles, you have time — use it to arrange a bigger tank or rehome extras.
Does the calculator include snails and shrimp?
Yes — cherry shrimp, mystery snails and nerite snails are in the list with small but non-zero loads, because a large snail produces more waste than most people expect.