A portable storage unit for storing fluid, particulates, or combinations thereof, wherein the portable storage unit can have a trailer with a frame. An outer container can be connected to the frame, and can have a first storage cavity with an outlet. A gate can provide for selective opening and closing of the outlet. An inner container can be movably disposed within the outer container, and can have a second storage cavity. The second storage cavity can be in selective fluid communication with the first storage cavity. At least a partial seal can be formed between the inner container and outer container. A hydraulic cylinder can be connected to the inner container and outer container for moving the inner container relative to the outer container, allowing for an adjustable volumetric capacity of the first storage cavity.