Whenever you ask what fish can fit in a tank, you want to think about the adult size of the fish. Let's say the full size of a fish is 2 feet long. You can put a fish like that in a 4 foot tank even though technically it would fit. It would cause the fish health problems and stunted growth because the fish wouldn't have room to swim. There is no exact rule about this, it is using common sense that can help make your decisions for you. Welcome to the site.