Aquarium stocking rules

All the worked examples below use one tank: a 20 gal long, 30 × 12 × 12 in. Gross volume is 30 × 12 × 12 ÷ 231 = 18.7 gal, and at 12% off it holds 16.5 gal of water. None of the rules below comes from controlled research. They are hobbyist rules of thumb that spread through pet shops, books, and forums because they are easy to remember. They are useful as a ceiling and unreliable as a target.

1 inch of fish per 1 gallon

The oldest and best-known rule. Add up the adult lengths of all fish, and keep the total under the number of gallons. In the 20 long that is 16.5 gal of water, so 16 in of fish (rounded down to 0.5 in), or about 10 neon tetras at 1.5 in. Limits: it ignores body shape and waste. A 4 in goldfish is far bulkier than a 4 in total of neon tetras. We use it only for freshwater community fish with an adult size under 5 in.

1 inch of fish per 3 gallons, for large and heavy-waste fish

Goldfish, plecos, and many cichlids make much more waste per inch, and they need space to turn. Tripling the water per inch is a simple correction. In the same tank at 6 in adult size: 16.5 ÷ 3 = 5 in, which is 0 fish. Limits: it still counts only inches, so a single 10 in fish can break a tank the rule says is fine.

Goldfish guidance: 20 gal, then 10 gal per extra fish

Common hobbyist guidance for fancy goldfish is about 20 gal for the first fish and 10 gal for each additional one. Applied to the nominal 20 gal size it gives 1 fancy goldfish for this 20 long, with no room to add a second, which is why many keepers pick a 29 or 40 gal. Single-tail goldfish (commons, comets) grow past 10 in and belong in 40+ gal tanks or ponds.

1 inch of fish per 5 gallons, for saltwater

Marine fish tend to be more active and territorial, and saltwater chemistry is less forgiving, so the usual saltwater convention allows one fifth as many inches as the freshwater small-fish rule. The 20 long set up as saltwater has 16.5 gal net, so 3 in of fish: one 3 in fish. Limits: some popular pairs, like clownfish, are kept in 20 gal despite the rule, which shows the rule is conservative, not a law.

Surface area: 12 sq in per inch, 30 for coldwater

Oxygen enters, and carbon dioxide leaves, through the surface. The tropical rule allows 12 sq in per inch of fish, and the coldwater rule 30 sq in per inch because goldfish use more oxygen. The 20 long has 30 × 12 = 360 sq in, which allows 30 in of tropical fish or 12 in of goldfish. Use whichever of the surface rule and the gallon rule gives the lower number.

Filtration adjustment

This one is ours, and it is not a published standard. We scale the result by 0.85 for low filtration, 1.0 for medium, and 1.15 for high. In the 20 long that moves 16 in to 13.5 in at low and 18.5 in at high. A good filter handles waste faster. It does not give fish more room, so keep the adjustment small.

Substrate and decor deduction

Sand, rock, and wood displace water. We take off 10%, 12%, or 15%. For the 20 long, 16.8 gal net at 10%, 16.5 at 12%, and 15.9 at 15%, which is 16.5, 16, and 15.5 in of small fish.

What matters more than any rule

Bioload, adult size, temperament, swimming space, and the minimum tank size of each species matter more than any inch rule. A pearl gourami is 4.5 in, so the one-inch rule says a 10 gal holds two, but they need a larger tank. A school of six neon tetras has a minimum of 6 fish before inches are counted. Stock slowly: add a few fish at a time to a cycled tank, and test ammonia and nitrite between additions.

Try the numbers on your own tank with the stocking calculator, or look at surface area, goldfish, and saltwater in more detail.