If you are thinking about fish that grow BIG and swim alot, such as triggers, then the best tank size would be 6, 8, 10 or more feet long. Distance for swimming is important and more important than gallons. Go into it knowing that it's difficult to sell, return or get credit for a large fish. There aren't enough people with space and desire for all of the big fish that are purchased small, outgrow tanks and need a home. A lot of rock, a good protein skimmer and good water circulation are all very good no matter what type of set up you may run.