How our numbers are worked out

The bioload model

Instead of the flawed 'inch per gallon' rule, every species carries a hand-tuned bioload factor — its relative waste and oxygen demand, with a neon tetra set to 1.0. A tank's capacity is its net water volume divided by a reference of about 4 litres per bioload unit, then adjusted by a footprint factor. Stocking level is the total bioload of your fish divided by that capacity, expressed as a percentage.

Why footprint matters

Two tanks of equal volume aren't equal. A long, shallow tank has more water surface for gas exchange and more floor length for swimming than a tall, narrow column, so it supports more fish. We multiply capacity by a footprint factor derived from the surface-area-to-volume ratio (a standard 30 cm²/L community tank sits at 1.0), clamped between 0.75 and 1.25 so it nudges the result rather than dominating it.

Filtration & water changes

Recommended filter flow ramps from about 4× the tank volume per hour when lightly stocked to 8× near capacity, because heavier bioloads need more turnover. The suggested weekly water-change percentage scales the same way. Both are planning guidance, not a substitute for testing your own water.

Net vs rated volume

Every result is anchored to net water volume — the rated tank size minus the water displaced by substrate and décor (often 10–15%) — not the empty glass capacity. Volumes are computed in exact units (1 US gallon = 3.785411784 L, 1 UK gallon = 4.54609 L) and only rounded for display.

How confident should you be?

Treat the outputs as well-reasoned estimates for planning, not guarantees. The bioload factors are conservative hobby-consensus values, individual fish and maintenance habits vary widely, and compatibility depends on temperament, layout and luck. Use the numbers to avoid obvious mistakes and to compare options — then research each species and watch how your fish actually behave.

Assumptions & limitations

The model assumes a cycled, reasonably maintained freshwater tank with adequate aeration and a single, accurate volume. It doesn't model live plants (which add headroom), exact filter media, breeding behaviour, or the specifics of marine systems. Saltwater species in the directory are provided for reference and aren't run through the freshwater stocking engine.

Sources & references

• · US and UK gallon definitions: NIST Handbook 44 and the UK Weights and Measures Act — used for exact volume conversions.
• · Species sizes, minimum tank sizes and water-parameter ranges: cross-checked against Seriously Fish species profiles and FishBase.
• · Filtration turnover and water-change guidance: consensus from established aquarium husbandry references and Practical Fishkeeping.
• · Surface-area / gas-exchange reasoning: standard aquarium science on oxygen exchange scaling with water surface, not volume alone.
• · Bioload factors: hand-tuned by our editors from the above sources and long-standing community stocking experience, normalised to a neon tetra at 1.0.