About & methodology

TankStockCalc is a free tool for checking whether a planned freshwater fish load is reasonable for a given tank, before you buy. It runs entirely in your browser — nothing you enter is sent to a server.

The stocking formula

Each fish’s bioload score is its adult length in centimetres multiplied by a species bioload factor (a modelling judgement, not a lab measurement, that scales typical waste output for the species). Total bioload is the sum of every fish row’s score times its quantity. Tank capacity is the tank volume in litres multiplied by a filtration multiplier (1.0 standard, 1.3 over-filtered, 0.7 under-filtered). Stocking percentage is total bioload divided by capacity, times 100. Full detail and reasoning is on the calculator page under “The methods explained.”

Separately, every fish is checked against its documented minimum tank size and, for schooling species, its minimum group size. Either check failing produces a hard-stop warning regardless of the percentage — these are physical constraints, not bioload math.

Species data & sources

Adult size, minimum tank size, and minimum school size for each of the 40 species in our dataset were checked against published aquarium references — primarily Seriously Fish, FishBase, and established aquarium retailer/society care sheets — rather than pulled from memory. Where sources disagreed, we used the more conservative (larger, safer) figure. Bioload factors are our own documented modelling choice, not a sourced figure.

Species not in our list can be entered manually via the “custom fish” option using adult length; the bioload math still applies, though we can’t run a species-specific minimum-tank or schooling check for fish we don’t have data on.

What this tool is not

It’s not a substitute for research on a specific species, a water test kit, or advice from an experienced fishkeeper. It covers freshwater only — no saltwater or reef systems.