I used to keep piranhas and the key, for those fish at least, was not necessary how many gallons the tank was, but rather length x width (amazonians like long and wide, with the height not being that important). Is the same true for salt water fishes. I wanted to get a smaller tank with a relatively large swimming area, so I got a 60g (48 x 15). I recognize one of the greatest benefits of a large volume tank is decreased variability in water parameters, but putting that aside, why else is volume important? Im not suggesting that I'm going to put tangs in my 60g (appeasing the tang PD), but what about keeping fishes that generally like a 90g tank (which I think has a 48 x 18 footprint)?