What kind of rack is it? And how big are your tanks? Are they 10gal tanks?
I had bought 2 types of racks (4 feet wide. one is 2 feet deep and another is 18 inch deep). They both claim they can support 1,500 pounds each shelf. Guess what, with four 10gal tanks (all are bare bottoms), the shelf starts curving. My 10gal tanks must be built with some outer space material and must be un-believably HEAVY.