How many fish will fit in your tank depends on the size of the fish. The best rule of thumb is 1 fish per gallon of water. This means that if you have fish that grow up to five inches, you can have 15 fish. If you have fish that only grow up to 3 inches, you could have up to 25 fish. Just make sure to keep an eye on your tank. Make sure that your fish are healthy and your tank is clean and clear.