Aquarium Size & Stocking Calculator

The 1-inch-per-gallon rule (modern version)

The classic guideline: 1 inch of adult fish per gallon of water. This works for small to medium community fish. Adjustments needed:

• Slim-bodied fish (tetras, rasboras, danios): 1 inch per gallon works fine.
• Body-mass fish (goldfish, cichlids, angelfish): Need 3-4× the gallons per inch — a 6-inch goldfish needs 20-30 gallons.
• Schooling fish: Always keep in groups of 6+. A single neon tetra is stressed; six together are calm.
• Bettas: Minimum 5 gallons, heated and filtered. The 1-gallon bowls sold at pet stores are inhumane.

Surface area matters more than volume

Fish breathe via oxygen exchange at the water surface. A tall column tank with the same gallon rating as a standard tank can only support 50-60% as many fish. Use this rule for tall/narrow tanks:

• Standard rectangular: Full 1-inch-per-gallon stocking.
• Tall column: 60-70% of standard capacity.
• Bowfront: 85-90% of standard capacity (slightly less surface than equivalent rectangle).

Standard tank sizes & dimensions

• 5 gallon: 16×8×10". Single betta + a snail. Nothing more.
• 10 gallon: 20×10×12". 6-8 small fish (neons, danios).
• 20 gallon high: 24×12×16". 10-15 community fish.
• 20 gallon long: 30×12×12". Same gallons, more swimming room — better than high for most fish.
• 29 gallon: 30×12×18". A common "step up" community tank.
• 55 gallon: 48×13×21". Good for medium cichlids or large community schools.
• 75 gallon: 48×18×21". Adult cichlids, multiple goldfish, larger angelfish setups.
• 125 gallon: 72×18×21". Serious tank — multiple oscar, plecos, large peaceful community.

Filtration sizing

Filter throughput should be 4-10× tank volume per hour:

• Low-stocked, planted tank: 4× turnover (40 gph filter on a 10 gal).
• Standard community: 6-8× turnover.
• Heavily stocked, big eaters (cichlids, goldfish): 8-10× turnover plus mechanical pre-filter.

Common overstocking mistakes

• Two goldfish in a 10 gallon: Need at least 30 gallons even as juveniles, 55+ gallons full grown.
• Six angelfish in a 29 gallon: Adults need 75+ gallons for a peaceful school.
• A single betta with neon tetras in a 5 gallon: Bettas often attack small fast fish. Keep bettas alone or with snails/shrimp.
• Plecos in small tanks: Common plecos reach 18-24". They need 75+ gallon tanks. Buy bristlenose plecos for tanks under 55 gallons.

For tested aquarium gear, see our pet reviews.