Stocking Calculator

Stocking Calculator

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Get a Stocking Starting Point

A rough starting point for freshwater community tanks, based on the classic surface-area rule – not a precise limit, and not a substitute for researching the specific species you want to keep.

cm
cm
Enter your tank footprint above
We'll estimate a rough freshwater stocking starting point.

Based on the surface-area rule: roughly 30cm² of surface per 1cm of adult fish length, combined across all fish. Surface area – not volume – is what this rule tracks, because oxygen exchange happens at the water's surface. This is a beginner starting point, not a scientific limit.

Treat this as a conservative starting point, not a target. This rule ignores tank depth, filtration capacity, water change frequency, live plants and – most importantly – the individual species you're keeping. A handful of small, slim-bodied schooling fish behave very differently to one large, messy-eating cichlid or goldfish of the same combined length. Always look up each species' adult size and care requirements individually – the size a fish is when you buy it is rarely the size it ends up.

FAQ

Why surface area instead of tank volume or litres?

Oxygen exchange happens at the water's surface, not throughout the volume – so a wide, shallow tank can support more fish than a tall, narrow one holding the same amount of water. That said, this rule ignores depth entirely, which is itself a real limitation worth knowing about – a taller tank gives your fish more swimming space and dilutes waste more even if the rule doesn't credit it for that.

Is this rule actually scientifically accurate?

Not really – and we'd rather tell you that than let the calculator imply otherwise. It's a legacy guideline from before modern filtration was common, and most reputable fishkeeping sources describe it as a rough, conservative starting point rather than an experimentally derived limit. Your real capacity depends heavily on filtration, water change routine, live plants, and which species you're keeping – some (peaceful, slim-bodied schoolers) can go well past this guide with good husbandry, others (large or messy species) should stay well under it.

Should I use the fish's size at purchase or its full adult size?

Adult size, always. Most fish sold small – plecos, many catfish, oscars, common goldfish – grow dramatically larger than what you bring home, and stocking based on purchase size is one of the most common ways tanks end up overstocked. Look up each species' realistic adult size before you buy.

Why isn't there a marine/reef version of this calculator?

Because we don't think a simple formula is honest for marine and reef tanks. Bioload varies enormously by species, system maturity and filtration/skimming capacity in a way that a surface-area number can't capture safely – see our marine guidance below instead of a calculator here.

Marine & reef tanks: we're not going to give you a formula here, because we don't think one can be trusted. Marine stocking depends on your specific species mix, system maturity, filtration and skimming capacity, and the order fish are added – far more than a surface-area number can capture. Add livestock slowly, research each species' adult size and temperament individually, and talk to our team in-store before adding anything you're unsure about. It's a conversation worth having before you buy, not after.
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