Well, the rating is 1000lbs per shelf.
A gallon of water is 8lbs so a 5 gallon tank is a approximately 40lbs and a 10 gallon is approximately 80. So it just depends on how many tanks you want on a shelf (you'll be lucky to fit 4 ten gallons if you place them sideways)
If the shelves are fiberboard, you'll eventually end up replacing it with plywood because it will get wet. And once it does, it will likely swell up and be uneven.
Also, if you have them in a fish room, the humidity will eventually cause the steel to rust.