If you are buying a small one around an inch or 2 a 55 gallon would be fine!! It isnt going to grow HUGE overnight!!! I would recommend just getting a 90 gallon though but it all depends on how much money you have bigger is always better!!! I mean why get a 125 if you can afford a 150 taller is better also with cichlids!! Just my opinion !!! 90 gallon is perfect for one or two Oscars to grow out!!!!!