I think what Pacu Mom is spot on. Typically, shimming needs to be ALL of the side, so to shim the front up, you want to either use one long piece that fits the whole front, or shim each foot. Good catch. And some opinions are that 1/4" is enough to be an issue with larger tanks.
Make sure to remove most of the water first. Lifting one edge of the stand at one time while the other edge is flat and the tank has a lot of water can potentially stress the seals on the tank.