Well, I guess I am going to try buying the Perfecto glass canopy for 18x72 tanks. It comes in two 18x36 sections basically. But I am a little confused about why the tank has that little inner lip/edge that is smaller than 18x36. That inner dimension would be like 17x35. I thought that was where the glass cover was supposed to sit. This cover would just go on the very top since it's too big to sit down on that inner edge. Maybe the inner edge is only if I am getting one of those black plastic hoods. I don't know what brand my tank is, I don't see any markings. But Perfectos do have the cross brace. The outer dimensions of the tank are actually slightly more than 72x18, by 1/2 an inch or so. Anyway I plan to sit my 72" long light on top of the glass cover too.