This can become a good discussion.
Fish normally have an unlimited supply of room but seems like lots of reef species become territorial, which claims so much of one particular area, he may never extend beyond that area for life. In a fish tank, your limited by room, how much area that can be claimed, by the smaller fish & how much swim room by the predators, which travel all over. So when you get a perfect balance is when you can keep all the fish you want & not have them eat each other then you have the right amount of fish, until they get bigger. Now with that being said, if you do fill your tank to that level, you have to develop a filtration system for the heavy feedings required & as much is fed, about the same becomes waste. I love Fish, this is the reason why I've been with tanks of water since I was a child, I had tanks with similar sizes with as much as 40 fish in a 20g tank, successfully. I also had One big fish in a 20g tank lol So how much water does a fish need?