KonvictKeeper
Aquarium Advice FINatic
IMO it's not the gallons, it's the footprint. A 55g is 48 1/4 x 12 3/4 x 21 (l x w x h). A 75g is 48 1/2 x 18 1/2 x 21 3/8. Everything is almost exactly the same besides the 75 is 18 1/2" front to back. An oscar can grow to be 15"+. If your fish is 16" long and your tank is only 12 3/4 front to back then he will have troubles turning around. If its 18 1/2" front to back he will have more than enough room to turn around. A 55g is more suited for a Jack Dempsey, as they grow to only 10" or so.