Freshwater Aquarium Tools

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.

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Tank Volume Calculator

Get exact gallons (and real water volume) from your tank's dimensions.

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Heater Size Calculator

Find the right heater wattage for your tank size and room temperature.

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How many fish in a...

Pick your tank size for real stocking examples and a calculator already set to it.

5gala betta with a snail, or a cherry shrimp colonyStocking guide10gala betta plus cleanup crew, or 8 ember tetras with corydorasStocking guide20gal8 neon tetras, 6 corydoras and a dwarf gouramiStocking guide29galan angelfish community, or a pair of Bolivian ramsStocking guide40gala pearl gourami with 12 tetras and 8 corydorasStocking guide55galtwo tetra schools, corydoras and a centerpiece gouramiStocking guide75galmultiple schools with a pleco, or 2 to 3 fancy goldfishStocking guide100galbig rainbowfish and cardinal tetra schools with a plecoStocking guide125gala common pleco, or an angelfish colony in dense plantsStocking guide

Popular freshwater aquarium fish

Neon Tetra
Neon TetraHolger Krisp · CC BY 3.0
Betta (male)
Betta (male)Daniella Vereeken · CC BY 2.0
Guppy
GuppyPer Harald Olsen · CC BY 3.0
Angelfish
AngelfishMattia Nocciola · CC BY-SA 4.0
Bronze Corydoras
Bronze CorydorasWikimedia Commons · Public domain
Boesemani Rainbowfish
Boesemani RainbowfishMattia Nocciola · CC BY-SA 4.0

Photos via Wikimedia Commons under their respective licenses, click a credit for the source.

New

Track your tank's nitrogen cycle

A new tank takes four to eight weeks to cycle, and the hardest part is not knowing whether what you are seeing is normal. Log your water tests and we will tell you which stage you are actually in, what comes next, and when it is safe to add fish.

Free, no account. Everything stays in your own browser.

Start a tank log
  1. NH3AmmoniaFish waste and food break down into ammonia
  2. NO2NitriteBacteria convert ammonia into nitrite
  3. NO3NitrateA second group converts nitrite into nitrate

Water changes remove the nitrate. Until nitrite reads zero, the tank is not ready for fish.

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.