yeah oscar gets too big and will outgrow a 55g within a year under the right conditions, i would suggest a GT if you can find a healthy one (this is harder than you would think), istlanums are nice fish though too VRWC has a pair and they are not afraid of you at all and hold their place, maybe a lyonsi, dont they stay small enough for a 55g and you get the look of a trimac, i would say midas which is always my first option for a wet pet but if its a male he would def need to be moved to a 75g