An improved mobile fluid storage tank having a base, substantially vertical front wall, a substantially rear wall, a pair of substantially parallel side walls and a roof structure. The roof structure has a rear roof portion and a front roof portion. The rear roof portion is substantially parallel with the base and extends along the rear portion of the two side walls while the front roof portion is comprised of a single sloping section which extends from the forward end rear roof portion to the vertical front wall. The intersection of the front and rear roof portions being selected to provide the optimum volume of fluid content of the tank.