Shim between the stand & the floor for the best results. Get a bundle of wood shims used for window & door installation, they cost about $1. Start with the front outside two if they are the lowest, bring it level front to back. Check across the front for level and bring up the low end, with the front & back legs on the low side. Once this is done you can wedge shims under any center legs to bring them solid with the floor.
Score the ends of the shims that are sticking out with a razor knife & snap them off for a neat appearance. Cardboard won't work, as it compresses, and will do nothing if it gets wet. Foam is OK for any imperfections with the top of a stand when dealing with a traditionally built tank as opposed to one with a floating base. Foam will do nothing to make a tank level, and certainly will not offer enough support under the legs due to being so compressible.
I've been dealing with this for years in my basement with a concrete floor that slopes towards a floor drain. Many tanks & racks are leveled with wood slats to bring them up to the point where I can adjust them with wooden shims.