AquariumCalcs: Fin & Figures — for the fish aficionado
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Aquarium Stocking Calculator

The old "inch per gallon" rule ignores fish shape, waste output and oxygen exchange. This uses surface area — a more realistic stocking method — and shows you the old rule alongside it for reference.

Your numbers
Only used to show the old inch-per-gallon comparison
Surface area

Add new fish slowly, not all at once

Add new fish in small batches over several weeks rather than all at once — your filter's bacteria colony needs time to catch up to each increase in bioload.

How the surface-area method works

Oxygen enters aquarium water mostly at the surface, through gas exchange, so the water's surface area — not its total volume — is the real limiting factor for how much fish life a tank can comfortably support. This calculator divides your tank's footprint (length × width) by a per-inch-of-fish factor that varies with how much waste and oxygen demand your fish produce: light for small, slim schooling fish, higher for larger or messier species like goldfish and cichlids.

Why the old "1 inch per gallon" rule falls short

The inch-per-gallon rule was never based on surface area, waste output, filtration capacity or fish shape — it treats a tall, narrow 20-gallon-high the same as a shallow, wide 20-gallon-long, and treats a slim neon tetra the same as a deep-bodied oscar. It's still widely repeated because it's simple, but it's at best a rough starting point, not a stocking limit.

What this number doesn't account for

Filtration capacity, water-change frequency, plant load (plants add oxygen and absorb some waste), and individual species' territorial or schooling needs all matter as much as the raw number. Research the specific adult size and temperament of every species before finalizing a stocking list, and always add new fish gradually rather than all at once.

Source: The surface-area approach reflects stocking guidance long recommended by professional aquarists and public aquariums in place of the informal "inch per gallon" rule, which has no real technical basis.

Frequently asked questions

Is the surface-area method actually more accurate than inch-per-gallon?

It accounts for oxygen exchange, which the inch-per-gallon rule ignores entirely, so it's generally considered a better starting point by experienced keepers. Neither method replaces species-specific research and watching your fish and water parameters over time.

Does a filter or air stone change how much I can stock?

Strong filtration and surfaceagitation (from a filter return, air stone, or powerhead) increase oxygen exchange and waste processing, which can support a somewhat higher stocking level than this calculator's baseline — but it's not a substitute for adequate surface area and regular maintenance.

Why does my bioload selection change the result so much?

A slim two-inch tetra produces a fraction of the waste and needs a fraction of the oxygen of a two-inch-per-day-growing goldfish. The bioload factor is doing the work of accounting for that difference in a single number.

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