I recently made a DIY fishtank stand for my new 20 gallon tank. I was careful to get straight pieces of lumber and precise cuts to make sure everything was square. The floors in my house are not level, so I went ahead and put notecards (I figure they work the same way as shims. I actually used a wood shim on one of the legs) under the legs to make the tank on top nice and level. I have filled the tank 1/3 full with water and everything looked nice and level, but there was a small gap between the plywood top and the plastic tank bottom--small, but you could see daylight between the tank and the stand (I have a standard 20 gal high glass tank). The corners all rested directly on the stand, but the middle of the long sides had a tiny gap. I went ahead and put styrofoam around the edges, but I'm wondering if this is a bad idea. So my issue isn't about the tank being level, but the top of the tank stand being perfectly flat/flush with the bottom of the tank.