How Many Fish Fit in a 20-Gallon Tank?
Calculate the maximum number of 2-inch fish a 20-gallon aquarium can comfortably support.
How to use this tool
- Enter your tank volume in US gallons (use the Aquarium Volume Calculator if needed).
- Enter the typical adult length of the fish species in inches.
- The result is the approximate maximum number of fish.
- Always research specific species needs — filtration, territory, and water chemistry matter more than this rule alone.
A 20-gallon tank gives you more flexibility with community fish — use this calculator to stock safely without overcrowding.
Frequently asked questions
- Is the one-inch-per-gallon rule accurate?
- It is a rough guideline only. Larger, messy fish (e.g. goldfish, cichlids) produce more waste per inch than small tropical fish. Always consider filtration capacity and species behaviour.
- How many fish can I put in a 10-gallon tank?
- Using the 1-inch-per-gallon rule with 1.5-inch fish: 10 ÷ 1.5 ≈ 6 fish. In practice, a 10-gallon is better suited to 5–6 small fish such as neon tetras.