Ok as promised here ya go
This is what I mean by having three nailed together. The top and bottom that they are nailed to is a 2x4
This is the drawing the lines indicate a 2 x 4. Hope this helps some. Wasn't sure how tall you wanted the stand and how wide a 125gallon is so I left them blank. The measurement on the front and back supports are different to keep them away from the doors.
I nailed plywood on top and all the way around it. But I wasn't going for looks because mine is in the wall. So when drywall went up they screwed to it. You may want to figure out how you can re use the finished wood and doors to cut back on some work. So you may need to mage some measurements around for the doors cause I didn't know how wide and tall they are. Best of luck to ya
Edit I forgot I didn't write down the 13 on the other end of the front. I was just going to tell you both ends are the same