TankStocked

Aquarium Stocking Calculator

Enter your tank size and the fish you plan to keep. You'll get an instant stocking percentage, an over/understocked verdict, a recommended filter flow rate, and compatibility warnings.

    Read me: This is a planning aid, not a guarantee. Real stocking capacity depends on your filtration, water-change routine, plants, feeding, and the specific temperament of your individual fish. Treat the percentage as a starting point and always research each species' needs before buying. When in doubt, stock lightly.

    How this calculator works

    Rather than the rough "one inch of fish per gallon" rule — which badly underestimates the waste of larger fish — this tool uses a body-mass-aware heuristic:

    • Bioload demand is summed across your fish using each species' adult length, weighted so that bigger-bodied fish count for more than slim fish of the same length (waste scales closer to body mass than to length).
    • Tank capacity is scaled from your tank's volume, which stands in for the usable surface area that drives oxygen exchange and filtration headroom.
    • Stocking % = bioload demand ÷ tank capacity. Under 100% is comfortable; 100–125% is heavy and needs strong filtration; over 125% is overstocked.

    The recommended filter flow uses the widely cited rule of thumb of turning over the tank volume about 4× per hour (so a 20-gallon tank wants roughly 80 GPH of real, in-tank flow). The warnings check tank-size minimums, schooling-group sizes, temperature overlap, and known aggression or fin-nipping conflicts.

    These thresholds come from common aquarium-hobby rules of thumb, not a single peer-reviewed standard. Use them to plan, then confirm with water testing once your tank is running.

    Frequently asked questions

    Is the "1 inch per gallon" rule accurate?

    Only loosely. It works okay for small, slim community fish but falls apart for larger ones: a single 6-inch fish makes far more waste than six 1-inch fish. This calculator weights larger fish more heavily, so its estimate is more realistic.

    What stocking percentage should I aim for?

    Beginners should target 100% or below. Above 100% can work for experienced keepers with over-sized filtration, frequent water changes, and stable parameters — but it leaves little margin for error.

    Does this work for saltwater or planted tanks?

    The numbers are tuned for typical freshwater community tanks. Heavily planted tanks can carry a bit more; reef and marine systems follow different rules. Treat results as a freshwater baseline.

    Related: How many fish fit in a 10/20/29/55 gallon tank? · Filter size calculator

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