How many fish a tank holds comes down to three things: the volume of the tank, how much of it substrate and decor take up, and how much water you allot to each fish. Enter those along with the number you keep today, and this tool gives the usable water volume, the number of fish it holds and how much room is left.
The math is (actual water x margin deducted) / water per fish, rounded down, and the formula used is printed below the results. There is no officially published stocking table by species. Recommended counts for the same tank size differ widely between sources, so this tool has no species list and instead asks for the per-fish allowance from the reference you are already using (as of October 2026).
This tool does not tell species apart, and it does not judge territorial behavior, compatibility between species or whether your filtration is adequate. Species that grow large need a much bigger per-fish allowance for the result to mean anything. This tool is for reference only and does not replace veterinary care. Contact your veterinarian or an aquatic animal hospital right away if your fish clamp their fins, hide or breathe rapidly.
Frequently Asked Questions
Use the figure in your own reference or the one your fish store gave you. There is no official table, so this tool asks for it rather than supplying one.
Enter 0 to calculate with no margin. A margin is worth keeping if the fish are still growing or the filtration is light.
No. This tool only divides volume and says nothing about compatibility or territorial behavior. Ask your fish store or veterinarian about mixing species.