FO to me means a tank would typically have fake coral, a FOWLR is fish with live rock with minimal or no corals. I think in a reef tank it is important to have at least 1 pound of LR per gallon, in a non-reef, I find plenty of swimming space more important, I have about 200 lbs in my 240, which allows my "big swimmers" (Angels, Tang, and Wrasses) plenty of area to swim.