Lastly, consider that any shims you do use will see tremendous crushing force PSI (pounds per square inch) and thus must be strong enough to sustain the point pressures of all that weight. (Foam is not an acceptable shim as that acts only as padding and easily compresses under the waters weight to where you started with in the first place.) To illustrate my point, take a piece of foam and place it on the floor and then stand on it. See if it holds you up or if it simply flattens out under the pressure and your weight. (In aquarium use, foam is a "feel good" material because it makes you feel good about using it but it does absolutely nothing.) In general,Plastic too should not be used for leveling a stand as most plastic can (and will) later crack or break down. Plastic may hold for a while but when you least expect it, it could fall apart later on which could then leave you in a precarious situation. Shims should only be solid wood or metal capable of sustaining the point weight pressures they will see. In general I have found that as long as your level is between the two lines, you should be OK once it settles in. If your aquarium is off a bit because your floor is off a bit, you are probably better off without shims at all as long as the level from end to end is less then 1/4" off, especially if you're talking about a 4 foot or longer aquarium. Just my opinion from my experience...