I wouldnt go over an 18" high tank. YOu would need really strong light, preferably MH if u did. That 50 sounds perfect 50x18x18. Nice amount of length width and height. If I had to add another tank right now, and keeping it not overly large, Id love to go 75 gallon but I would never in an apartment that I may move out of within a year or two, ID go with a 50 gallon.