A tank is formed from panels fastened together edge to edge to form four upstanding side walls and base panels where across the base the panels extend from one side to the other. The panels are composite and formed from a honeycomb core panel with a foam material filling the tubular cells and a fibrous reinforcing cover sheets. The walls are supported by a metal frame including a base member extending along the wall at the bottom of the panels and a plurality of joining members connected to the base member and upstanding therefrom for holding the ends of the panels in end to end relationship. Cables extend along the walls adjacent the base member and connect to end posts of the walls. A series of posts stand along the walls on the outside and are connected across the tank by cables extending over the top edge of the walls.