Sue, I would also think however that you need to consider the additional amount of water exposed to the air at the surface as a potential contributor to evaporation. The reason we aerate is to move water so that more of it comes in contact with the atmosphere. This fact alone could drastically increase evaporation beyond your calculations which only consider how much water the pumped air will take up. I think you're missing a large part of the impact of aeration on evaporation. Water that is moving in contact with the atmosphere will no doubt evaporate faster. Don't know the magnitude but this would be an interesting study.

BTW, a side question. Another consequence of the hot weather is that many of us have more problems with various floating plants such as DW. Does the presence of DW help retain water or does it speed up evaporation in some way?

Gotta get back to fishin!