I've made several 36 x 24 x 8" frag tanks and those are pretty solid. 48" might be OK, but you could start seeing some bowing over time. You may not need a full perimeter euro, at least a strip along the front/back edge. Though a perimeter euro does look better, if you make it one piece. But that takes up 1/4 sheet of material. So if you are not going for looks, just functionality, a 1.5" to 2" strip would help. It may not be completely necessary.
Let's put it this way: if the tank is well built, if you do have a failure, it's probably not going to be a catastrophic seam failure. You will *probably* see the signs well ahead of time...
