Thoughts: One of two things or both are going on and I would not fill a glass box with saltwater in my house without fixing them first. 1: The tank is of poor construction and is way out of square, not likely but could happen. 2: Your stand is way out of wack and needs to be fixed or replaced. Either way you should not have to "shim" the tank between the tank and the stand, shimming should only be a fix for an uneven floor.