Abstract: A storage tank, principally for fluids, having an inner primary tank formed of steel, and having a plastic coating, and an outer tank formed of sheet material composed of glass fibers and plastic resin, and having a coating of glass fibers and resin applied to the exterior thereof. Elongated spacers formed of plastic material and having a generally semi-circular, arcuate cross-section are positioned between the cylindrical wall of the primary and outer tanks as well as between the corresponding endwalls thereof. The elongated arcuate spacers beneath the primary tank are in closer spaced relation to each other than are the elongated arcuate spacers above the primary tank. A plastic elongated probe member is positioned between the adjacent endwalls of the primary and outer tanks to be positioned vertically when the storage tank is properly mounted in a horizontal position, to enable tests to be made within such probe member of fluid material which may accumulate between the primary and outer tanks.