If I understand your set-up, you have a OSB board laying on the carpet, with the stand sitting on that board, and the tank sitting on the stand. I wouldn't even try putting shims between the board and the carpet - they would gradually sink into the carpet. I would try to put shims under the stand, on the board. If the stand has feet, you need to shim just the feet. But, if the stand doesn't have feet, you will do much better with a strip of shims across the front, under the stand front edge, since that is low. I have had this problem too, and finally gave up on the board, just accepting that the stand will leave a visible compressed mark on the carpet. Then I shimmed between the carpet and stand, with long strips of shim. I found that strips of wood veneer worked best as shims. I hope you plan to remove most of the water from the tank before shimming, so you don't have to lift the full tank to install the shims.