How many fish can your tank actually hold? This free fish tank stocking calculator uses the classic 1-inch-per-gallon community guideline to give you a general starting point.
This rule works best for small-to-medium community freshwater fish – large or messy species need more room per inch.
This is a rough general guideline, not a substitute for species-specific research. Filtration, tank shape, and fish behavior all matter too.
What This Fish Tank Stocking Calculator Shows You
A fish tank stocking calculator applies the classic “1 inch of fish per gallon” community aquarium guideline, giving a rough starting point for how many small fish a given tank size can generally support.
Why This Rule Is a Starting Point, Not a Hard Rule
This guideline was designed for small, peaceful community fish in a reasonably filtered tank. It doesn’t account for adult fish size (rather than size at purchase), waste production differences between species, or territorial behavior.
Fish That Break the Rule Badly
Large or heavy-waste species like goldfish, and territorial fish that need significant personal space, require far more than 1 inch per gallon – always research a specific species’ actual needs before stocking.
Filtration Matters as Much as Volume
A well-filtered tank can often support slightly more stock than this rule suggests, while an under-filtered tank may need less – filtration capacity and regular water changes are just as important as raw gallon count.
Stock Gradually, Not All at Once
Adding fish gradually over several weeks lets your tank’s beneficial bacteria colony grow to handle the increased bioload, reducing the risk of an ammonia spike that can harm or kill fish.
Check other pet math with our Dog Food Portion Calculator or Pet Age Calculator.