Cooling a Hot Pool

Contrary to Matt's suggestion of running the pump at night to mix the water I would suggest that maybe the opposite would work.

If the water is more or less stagnant, the hottest water will rise to the top. You can ofter feel this when you swim in stagnant water. Colder at the bottom.

So if you want to lose heat, maybe you should let it rise to the surface and cause more evaporation for cooling. Everyone I know says the greatest loss of heat when you're trying to get a pool warmer, is off the surface.