Is there water in it? Is it on carpet, wood, or concrete.
It sounds like the stand is on the ground with an empty tank?
If this is the case, the wood could be crowning and bending it up. Or just plain out of square. I would imagine with water in it, it will level out.
Or is it the floor that is out? Then shim it using door shims or I used cardboard on one to level it out, but that moulds and smells.
Your screen name is Nova67, which I assume you like and work on cars.....think gasket and how to mate two uneven surfaces.
Remember pure water wieghs about 8.34 pounds per gallon, and salt even more(not going to look it up), so at 135 thats 1126 pounds of weight pushing down on it. I don't know about you, but I would flatten out at that weight.