Yeah the casters are rated for 750lbs I believe, which should be more than adequate for the empty tank load evenly spread, however when I rolled it across my driveway (which is that bumpy stone finish) to take the pics a little while ago, it was far from smooth and easy gliding and I was thinking the same thing so I did a little research and I think we should change out the casters to pneumatic tires. I pitched the idea to my husband in an email so we'll see what he says... he can be stubborn. Lol
I am not concerned about the frame holding the weight. Remember it is going to be empty, and I have seen stands that weren't even built this strong, and while they could fail over time, the tank will only be on here with it's empty weight and for not very long at all. I'm not sure how much a glass tank this size weighs but we've guessed around 400- 500 lbs. My husband is a big, solid guy and he got on the cart in the center where there are no wheels and jumped in it over and over and it was no problem for it. He only weighs a bit over 200lbs but the tank will have it's weight spread equally over the cart so it should be fine. He's very smart with stuff like this and great with building things right. My only real concern is the wheels. I feel it's a must to change them out.