The Inch-Per-Gallon Rule (and Its Limits)
The classic "1 inch of fish per gallon" rule is a simple starting point but has well-known limitations. It doesn't account for fish body mass (a 6-inch oscar produces far more waste than six 1-inch tetras), fish temperament, bottom vs. top vs. middle water layers, or territory needs. Use this calculator as a maximum guideline, then monitor water parameters to confirm your tank isn't over-stocked.
Why Filtration Matters for Stocking
A heavily filtered tank (like a planted tank with a large canister filter) can comfortably handle 20-50% more fish than one with a basic hang-on-back filter. The nitrogen cycle — where beneficial bacteria convert toxic ammonia into nitrite, then into less harmful nitrate — is what makes higher stocking possible. A mature, well-filtered tank is significantly more forgiving.