Aquarium Size Calculator

Minimum Gallons is built from the classic "inches of fish per gallon" aquarium rule of thumb: Number of Fish times Average Fish Size gives total fish inches, then that figure is multiplied by a gallons-per-inch factor that varies by Fish Type — 1 gallon per inch for tropical fish, 2 for goldfish and cichlids (which produce more waste and need more swimming room than typical tropical community fish), and 5 for marine fish, whose bioload and water chemistry sensitivity is much higher. A per-species floor is applied on top of the per-inch figure — 10 gallons for a tropical community tank, 30 for goldfish or cichlids, 20 for marine — because a single large fish sized on inches alone lands well below every published species minimum. Single-tail goldfish (common, comet, shubunkin) need 40 gallons for the first fish and realistically belong in a pond; the 30-gallon floor here reflects fancy varieties. Enter adult sizes: the inches-per-gallon rule prices the fish a tank has to hold at full growth, not the size they arrive at. Both Number of Fish and Average Fish Size multiply directly into total fish inches, and they carry exactly equal weight: doubling either one doubles Minimum Gallons.

Ten four-inch fish and forty one-inch fish produce the identical 40-gallon figure under this rule — which is one of the rule's known weaknesses, since forty small fish and ten larger ones do not in practice place the same demand on a filter. Recommended Gallons adds a 25% safety margin on top of Minimum Gallons, slightly reduced when Live Plants is set to Yes since live plants improve biological filtration capacity. Filter Rating sizes the pump at four times the recommended volume per hour for freshwater tanks and ten times for marine, and Heater assumes 5 watts per gallon — the conservative end of the usual 3-to-5-watt band, appropriate for a room that runs cool. Heater is set to zero specifically for goldfish, since they are a coldwater species commonly kept without supplemental heating — tropical, cichlid, and marine setups all get a heater wattage recommendation, since all three groups need stable warm water. This rule-of-thumb approach is a simplified starting point; species with unusually large adult sizes, territorial behavior, or specialized water chemistry needs may require more space than the inches-per-gallon guideline suggests.