Leveling an Aquarium stand is one of the most critical steps. If you remember, we made the legs exactly the same length and then built the stand to hold them there. Due to some wood warping, I had to level the aquarium with 30-gallon tubs of water before I put the cabinet and bracing in place.
The cabinet and bracing hold everything in place, but before those are installed, we need to ensure the stand is in its final resting spot and everything is square. I had some warping of the boards I used, and two feet were a little above the floor.
If I had put the cabinet and bracing on at this point, it would have locked the legs in an un-level position that was not square. This causes many problems and could lead to the stand or the tank failing catastrophically.
So, before I put the cabinet & bracing in place, I put two 30 gallon tubs of water (~250 lbs each) on the stand, which leveled it out nicely. Then, I put the bracing and cabinet in-place to lock it in that position. All is well.