Calculator
Reef Stocking Calculator
"How many fish can my tank hold?" has no exact answer — but it has a sensible range, and some hard rules about who fits where. Set your tank size, build a fish list, and get a bioload score against a conservative capacity — plus the minimum-tank and aggression warnings that actually sink stocking plans.
Not sure of your real volume? The tank volume calculator gives the true figure after rock and sand.
Build your list
Set a quantity for each fish you're planning.
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How the score works
Each fish contributes a bioload from its adult size and how messy an eater it is — a tang puts out far more per inch than a goby. We total that against a capacity calibrated to real well-stocked reefs — not the old inch-per-gallon myth — scaled to your true water volume and adjusted for your filtration. Land in the well-stocked band and you've got a full, stable reef; go over and you're signing up for more water changes and more squabbling. It's a planning guide, not a hard limit — a lightly-fed tank with a big skimmer carries more than the number suggests.
The rules that aren't fuzzy
Bioload is an estimate; minimum tank size and aggression aren't. A yellow tang needs horizontal swimming room no 40-gallon provides, two dwarf angels will fight to the death in most tanks, and a six-line wrasse turns into a bully as it matures. Those warnings fire regardless of the load score — they're where stocking plans actually fail. Stock slowly, add the feisty fish last, and quarantine.
The assumptions (flagged, as always)
Adult sizes and minimum tanks are hobby consensus (LiveAquaria / Reef2Reef), not lab data — individual fish vary. Bioload factors are relative estimates, not measured waste output. The capacity line is deliberately conservative; experienced keepers with heavy export push past it. Fish list updated 2026-07-12. This tool won't replace researching each species before you buy — it catches the obvious mistakes early.
Stocked on paper? Size the gear.
A full fish list needs a skimmer that keeps up. The skimmer sizer derates the optimistic ratings for your bioload, and the skimmer rankings pick the right one by tank size.
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The Reef Gear Cheat Sheet.
The exact skimmer / pump / light picks by tank size, plus our full build-cost worksheet — as one email, no spam.
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