How many fish can your tank really hold?
A species-aware stocking calculator that checks bioload, schooling needs, temperament, and water parameters — not the myth of "one inch per gallon." Free, no signup.
Stocking Calculator
Build your stock list and see your true stocking level plus compatibility warnings.
Open toolTank Volume Calculator
Get exact gallons (and real water volume) from your tank's dimensions.
Open toolHeater Size Calculator
Find the right heater wattage for your tank size and room temperature.
Open toolHow many fish in a...
Pick your tank size for stocking ideas and a calculator pre-set for you:
Popular freshwater aquarium fish
Photos via Wikimedia Commons under their respective licenses — click a credit for the source.
Frequently Asked Questions
Is the "1 inch of fish per gallon" rule accurate?
No. It ignores body shape, waste output (bioload), activity level, territory, and adult size — a goldfish and a neon tetra are nothing alike. FishAuthority uses a species-aware bioload model with compatibility checks instead.
Do I need to cycle my tank first?
Yes. A fishless nitrogen cycle (building beneficial bacteria) takes a few weeks and is the single biggest factor in keeping fish alive. Stock gradually after the tank is cycled.
Is this calculator free?
Completely free, no signup. We earn a small commission if you buy gear through our links, at no extra cost to you.