Metal or wood? Its hard to offer ideas without a description or picture. If wood, maybe you could use a wood filler like Plastic Wood to build up the low area. If it’s a metal stand, maybe epoxy or JB Weld. Goop some on, cover with wax paper and smooth it level with a putty knife or set the tank on it to level it. When hardened, remove wax paper and sand it to match the contour of the metal. All just general guesses without seeing what you have.