Thats a very good question. When i was doing my tank transfer i broke(accident) the plastic center brace on my 75 and was hesitant to finish the swap. I added the sand,rock, and the water and then waited. Everything seemed ok so i added the CUC and livestock. The next day I notice a little bowing and measured. It was 1/4 of an inch so I left it alone. Its been a month now and its the same measurement. I may try to fix it but I dont know how.