Aquarium stocking calculator
Pick your tank and fish: we draw them at adult size and check every pair.
For the example fish: No conflicts found. The fish drawn to scale ↓
Your stocking check
Worked example fish
No conflicts found
No conflicts found in the checks below.
Water readings missing: temperature, pH, GH. Shared species ranges do not tell us whether your water matches.
The surface check uses the named tank’s outside length and width. Enter inside measurements for your actual footprint.
Actual water volume is unknown; the minimum-size check uses the tank’s label gallons.
Swimming clearance, filter capacity, cycling, territory and male-to-female ratios are not checked here. Test ammonia and nitrite before adding fish; matching these ranges does not show that a tank is cycled.
20 gallon long
30¼ × 12½ × 12¾ in outsideDrawn to scale at adult size. Waterline is schematic.
Your tank and water
Minimum tank sizes are checked against the tank label.
Shared species ranges
The Merck Veterinary Manual’s freshwater guideline: about 1 inch of adult fish per 12 square inches of water surface. It is a ceiling, not a target.
Next: plan weekly water changes · browse filters · more fish by tank size
Find fish for this tank
Species that suit the tank, water and fish in the check above, each with the reasons. Add one and the check updates.
Tanks are matched by US gallons, as the stocking checker does: none of the 252 species here has a known minimum floor space, so length and width are not checked. Blank readings are not checked. Tank mates use the stocking checker’s pair checks; temperament alone does not make a group work.
How the check works
The checker draws your fish at adult size in your tank and checks each pair against the care facts: tank size, temperature, water, group size and temperament. Then it adds up the fish’s adult length and compares it with the tank’s water surface. It is a guide from those facts, never a score.
Every species here has the same care facts: adult size, the smallest tank that suits an adult group, temperature range, pH and hardness, temperament, minimum group size and a few behaviors (nips fins, eats shrimp, eats snails, needs its own tank). The checker compares each fish with your tank and with every other fish you add:
- Tank size: each species' minimum tank against your tank. Inside measurements give an upper volume before gravel and decor; measured water, if entered, can lower that volume.
- Temperature and water: shared ranges describe the species. Your current readings are checked separately against each species; missing readings stay unknown.
- Group size: schooling fish below their group minimum get a note to add more.
- Temperament: predators against fish small enough to eat, fin nippers against long fins, shrimp and snail eaters against shrimp and snails.
- How many: the adult length of all the fish added up, against the Merck Veterinary Manual’s freshwater guideline of about 1 inch of fish per 12 square inches of water surface. Custom measurements use inside length and width; named tanks use outside dimensions as an upper limit. Shrimp and snails are not counted.
The surface guideline is a ceiling, not a target, and it does not make a "percent stocked" score: the old "one inch of fish per gallon" rule only works for small, slim fish, and Merck notes that a tank’s real capacity depends on its filter and on the room each fish needs. Start lightly, add fish in small groups to a cycled tank, and test ammonia and nitrite after each addition.
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