i didnt want gallons because in new zealand where i am from we use metric stuff not weird american measurements , it just to confusing... i can convert the inches into centermeters and go from there. thanks heaps
Although a larger tank than a 55 gallon is better. I've kept an oscar in a 55 gallon and would now recommend a 75 gallon. That's probably close to 300 liters. Or closer to 48cm deep x 58 cm tall x 133 cm wide or 18" deep x 21" tall x 48" wide. Those should be fairly close to the American 75 gallon.
I am now keeping two in a 600 gallon tank. 84x40x40 and the two are already taking over the tank. They are just under 4 inches long. Volume does not matter IMO. Its more on length and overall dimentions. Bigger is better, not so you can stock more but give these fish some room to grow and show more of their natural behaviors. My fish just act different in my big tank than they do in a small one. Much less agression as well. Plenty of room to get away if need be. Mine are fighting over the Food rock glued to the center brace. They like to hide under it and ambush the pellets that hit the water like dropping bug bombs from heaven.