Aquarium Bioload Calculator
Gauge how heavily your freshwater tank is stocked with the inch-per-gallon guideline, adjusted for filtration, and see your remaining capacity.
How it works
Capacity (in) = tank gallons × in/gal guideline · Bioload used % = stocked inches ÷ capacity × 100
The inch-per-gallon guideline is a rough proxy for bioload — the waste burden a tank’s filter and water changes must handle. It works best for small, slender community fish and breaks down badly for heavy or messy species (a single goldfish or a plecostomus outweighs its inches), so treat it as a starting sanity check, not a hard limit. Better filtration and frequent water changes justify the higher factor, while sensitive fish call for the conservative one.
Worked example
A 20-gallon tank at the standard 1.0 in/gal supports about 20 inches of fish. Six 1-inch tetras plus a pair of 2-inch platies is 10 inches — 10 ÷ 20 = 50% of capacity, lightly-to-moderately stocked.
Frequently asked questions
Does the inch-per-gallon rule really work?
Only as a first approximation for small, thin-bodied community fish. It ignores body mass, waste output and territory, so a "12 inches" of goldfish overloads a tank that "12 inches" of neon tetras would barely tax. Use it to catch obvious overstocking, then research each species.
Should I use tank volume or actual water volume?
Actual (net) water volume is more accurate, since substrate, rocks and equipment displace water — a "20 gallon" tank often holds 17–18. If you want to be careful, subtract 10–15% from the labelled size before calculating.
How does filtration change the number?
A tank with strong biological filtration and regular water changes can safely carry a higher bioload, which is why the heavily-filtered setting allows more inches. Weak filtration or infrequent maintenance pushes you toward the conservative factor.
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Educational estimate only — not veterinary advice. Always consult a licensed veterinarian before dosing, treating, feeding, or making care decisions for any animal.