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Enter tank volume in gallons, the adult length of the fish (not the juvenile size at purchase), filtration type, stocking philosophy and whether the tank is planted. The calculator applies the inch-per-gallon rule with modern adjustments for filtration and plants, then gives maximum fish inches, maximum fish count and the minimum filter flow in gallons per hour. Always use adult size: a 2 inch juvenile oscar can grow past 12 inches. These are planning estimates, not a substitute for species research.
The classic inch-per-gallon rule allows one inch of adult fish per gallon of water. This calculator refines it: maximum fish inches = tank gallons times a stocking rate times a filtration multiplier times a planted bonus. Stocking rates are conservative 0.5, moderate 0.75 and standard 1.0 inches per gallon. Filtration multipliers are sponge 0.8, hang-on-back 1.0, canister 1.2 and sump 1.4, because better filtration processes more waste and supports more bacteria. Live plants add a 1.15 bonus, since plants absorb nitrates and add oxygen, but plants alone cannot fix severe overstocking. Minimum filter flow is 4 times tank volume per hour (GPH), a turnover rate of 4 times per hour, rising to 6 to 10 times for heavy stocking. Together these give stocking density in inches per gallon.
Enter tank volume (for a rectangular tank, length times width times height in inches divided by 231 gives gallons), adult fish length, filtration type, stocking philosophy and planting. Beginners should choose conservative or moderate; experienced keepers with disciplined maintenance can consider standard. Read maximum fish count as an upper limit, not a target, and stock new tanks at 30 to 50 percent of capacity first, adding 2 to 3 fish at a time with at least two weeks between additions so the biological filter can adjust.
Surface area governs oxygen exchange at the water surface, so long shallow tanks support more fish than tall narrow tanks of the same volume. The surface area rule allows 1 inch of tropical fish per 12 square inches of surface (1 inch per 30 square inches for goldfish and coldwater fish). A 20 gallon long tank therefore often supports more fish than a 20 gallon tall tank. If your tank is tall and narrow, treat this calculator result as optimistic and drop one stocking level. Goldfish break the rule entirely: allow 20 gallons for the first fancy goldfish and 10 gallons for each additional one.
Example 1, a 20 gallon community tank with neon tetras at 1.5 inches adult, hang-on-back filter, moderate philosophy and plants: maximum inches = 20 × 0.75 × 1.0 × 1.15 = 17.25 inches, so maximum tetras = 17.25 / 1.5 = 11 fish, with minimum filter flow 80 GPH. Example 2, a 55 gallon cichlid tank at 4 inches with a canister filter and standard philosophy, no plants: 55 × 1.0 × 1.2 = 66 inches, so 16 cichlids. Example 3, a 10 gallon beginner tank with 2 inch guppies, sponge filter and conservative philosophy: 10 × 0.5 × 0.8 = 4 inches, so 2 guppies.
A 10 gallon tank typically holds 5 to 12.5 inches of fish depending on philosophy and filtration; with moderate stocking and a standard filter, about 7.5 inches, such as five 1.5 inch neon tetras or a single betta with a few tankmates. A 20 gallon tank typically holds 10 to 25 inches of fish: 10 to 15 small fish such as neon tetras, guppies or rasboras, 5 to 7 medium fish such as platies, or 2 to 4 larger fish such as dwarf gouramis. A 20 gallon long tank suits schooling fish better than a tall tank, because it gives more horizontal swimming length.
Overstocking shows as poor water quality despite maintenance, aggression and territorial stress, clamped fins, rapid gill movement, lethargy, stunted growth, frequent disease and algae that will not clear. Fish in overstocked tanks also face lower oxygen at night, when plants stop producing oxygen. If you suspect overstocking, increase water change frequency (25 percent weekly is a good baseline), upgrade filtration, add live plants, and rehome fish if needed. When in doubt, understock rather than overstock: fewer fish with adequate space are healthier, more active and longer lived.
It is the traditional guideline of one inch of adult fish per gallon of water: a 20 gallon tank could hold 20 inches of fish. Modern keepers refine it with filtration, planting and body mass adjustments, which is what this calculator does.
Typically 10 to 25 inches of fish depending on stocking philosophy and filtration. With moderate stocking and a standard filter, about 15 inches, such as five 3 inch platies or eight 2 inch neon tetras, or 11 neon tetras in a planted tank.
About 5 to 12.5 inches of fish, or roughly 8 to 10 small community fish such as neon tetras, or a single betta with a few tankmates. Small tanks are less forgiving of overstocking, so beginners should choose conservative estimates.
Stronger filtration increases biological capacity by processing ammonia and nitrite more efficiently. A basic sponge filter supports about 80 percent of standard stocking, a canister about 120 percent, and a sump about 140 percent. Filter flow should turn over at least 4 times tank volume per hour.
Yes. Live plants absorb nitrates, provide surface area for beneficial bacteria and add oxygen, so planted tanks can support roughly 10 to 20 percent more fish. This calculator applies a 15 percent planted bonus, but plants do not replace filtration or water changes.
Yes, but consider adult size, temperament and bioload for each type. Community fish, cichlids and goldfish have very different needs, so run the calculator for each type and check temperature, pH and temperament overlap before mixing.
Surface area controls oxygen exchange at the water surface. More surface area allows more oxygen, which supports more fish safely, which is why long shallow tanks outperform tall narrow tanks of equal volume.
Good filtration helps, but it does not remove the need for swimming space, territory and water changes. Even with a sump and plants, territorial species and heavy-bodied fish need space per fish, not just clean water.