A tank assembly includes a tank having a plurality of tank walls surrounding an interior cavity. The interior cavity is configured to store a fluid. The fluid is configured to expand upon a change in temperature. A fill channel and a vent channel are each in fluid communication with the interior cavity. At least one compressible member is positioned within the interior cavity and configured to provide an expansion space for the fluid. The compressible member eliminates the need to be concerned with a loss of air space when filling the tank with low flow rates and when filling on grades. The compressible member may be composed of a closed cell foam material. Alternatively, the compressible member may include a pouch having a hollow space surrounded by an outer skin.