A portable fluid storage system for use during frac jobs or other operations requiring storage of water or other fluids. The system comprises a lower chamber and an upper chamber sized to fit within the lower chamber. A lift system, such as screw jacks or rack and pinion gears, is disposed to move the lower chamber from a collapsed position to an extended position where the upper chamber is positioned above the lower chamber. A seal is disposed to engage the upper chamber and the lower chamber when the system is in the extended position. The upper chamber, the seal, and the lower chamber form a water tight compartment when the system is in the extended position. The system is transported as a traditional frac tank with a tractor or other truck. When positioned for use, the storage system is expanded to hold twice as much fluid as a traditional tank.