My 55 gallon leaned toward the left about 1". started affecting the partical board stand too after a few months. One night I was hearing cracking noises from behind the stand.
Next day I drained the tank half way and lifted one side up while I had someone put some wood under the stand.
My 30 is the same way. Guess thats what you get for living in a barn.
I used a leveler and all my tanks are pretty level. I don't know what millimeter they're off but they're well within safety parameters. I always make sure before and after adding water that they're as balanced as possible.
All mine are out too the 4 foot one is about 5mm out, so not bad at all.
I do intend to level it up, not because I'm worried about the structural integrity, just because I don't want to see the waterline. I might even do it this weekend .
When I put in the 125 usg, the lfs man was very careful about it all. I was trying to put the fory glass tank half on carpet and hlf on the tile. He gently redirected me. I cracked the 100 usg just trying to force a filter in. I is ok, but I understand that it is an important issue. My 60 usg on the porch with the goldie slants forward with the porch drain slant. This thread has me thinking about shimming it. It is at least an inch drop from back to front--maybe more.
If you can do something to level the tank, I think it would be wise.
I just noticed from proping up one side of the fish tank stand on my 55 gallon. Now the entire stand is bowing in the middle so ONLY the left and right sides of the tank are being supported. The Entire middle of the tank is not. I need to find a way to fix this....