If it's manufactured I would contact the maker and see if they can tell you. A 20 gallon tank will end up around 175lb. Different woods have different load capacities and a lot of that is determined by how the peices are joined and what with. If it's not manufactured, give a little more info here and I'm sure someone can tell you.
30 Gallon FW. 1 Albino Bushynose Plecostomus, 2 Harlequin Raspboras, and 1 Male German Blue Ram