[matm347]

The reservoir design can aid in the performance of a WC system, but more attention should be given to the radiator, where the majority of heat transfer will take place.

But with a decent WC setup, the water shouldn't be above ~60C. The environment inside the reservoir wouldn't be a very volatile one since there is no heat being added externally. Granted, there will be turbulence due to the flowing of the pumped water, but not much else will be going on. Room for improving the thermal properties of a reservoir is somewhat limited, simply apply what you know and increase the surface area using a good conductor of heat as the material.