A downhole apparatus comprises a casing string with a degradable plug therein to block flow therethrough. A flow barrier is positioned in the casing below the degradable plug and the degradable plug and the flow barrier defining a buoyancy chamber therebetween. An annular fluid reservoir holds a degrading fluid. The fluid reservoir has first and second rupture disks at upper and lower ends. Fluid from the annular fluid reservoir is communicated to the degradable plug after the first and second rupture disks rupture.