The Stocking Rule

"Stocking" is planning the population of a tank so the fish have space, the filter can cope with the waste, and temperaments don't clash. Getting it wrong — overstocking — is one of the most common causes of poor water quality, stress and disease.

The old rule of thumb. The familiar guideline is one inch of fish per US gallon (roughly 1 cm per litre) of water. It is a convenient starting point for small, slender community fish but breaks down quickly. It ignores that a single 6-inch-long, deep-bodied fish produces far more waste and needs far more space than six 1-inch tetras, and it ignores swimming and territorial needs entirely.

What really matters. Better stocking weighs several factors: the fish's adult size (not its size in the shop), its waste output (messy fish like goldfish and large cichlids need far more room and filtration), its activity and swimming needs (fast swimmers need length), its social needs (schooling fish need groups, which counts toward the load), and temperament and compatibility (aggressive or predatory fish limit what else fits).

A safer approach. Under-stock rather than over-stock, especially while learning; add fish gradually so the biological filter can adjust; and plan around adult sizes and full schools from the start. Use the inch-per-gallon idea only as a loose sanity check for small community fish, and lean on species-specific minimum tank sizes and your own water-test results as the real guide. A lightly stocked tank is healthier, more stable and far easier to keep than a crowded one.