use hardwood shims if you need to level the stand. use enough to get full contact across the entire bottom surface of the stand.
how much out of level are we talking about? over the length of your tank, how much of an elevation change is there in the floor?
also, how many screws did you put in each leg where they attach to the top and bottom? with the garf stand design, you are relying on the screws to transmit the load from the top frame to the legs.
i would put a full sheet of plywood in on the top of the lower frame. if you dont, the first time something rolls under that plywood you are going to kick yourself for not enclosing it. also, i would add a sheet of 3/4 plywood to the bottom of the bottom frame.
~mike