I had a rack built to hold 48" tanks. The top shelf was actually 52" IIRC and I wanted to put 3 10s across that top shelf, which if you know your aquarium dimensions would mean the shelf was just a tiny bit too small. To compensate I cut a piece of plywood to match the footprint of the 3 10s (either 5/8" or 3/4" - I forget). The tanks were setup for ~4 years until I took them down to move and I never had any problems.