I was actually wondering if I need to worry about summer heat yesterday. Since the liner on the waterfall stream is black, I'm not sure if the waterfall will do much to actually cool the water. The pond was quite warm (at least at the surface) yesterday and it was only in the 70s.

I should have a lot of surface coverage of plants when my water lilies finally kick into gear and start really growing, so that may help too.

I use air stone's year round in my pond. In the winter they keep a hole in the ice and in the summer they help circulate the water from the bottom of the pond and help add oxygen when it gets really hot. They are cheap and good to have. I actually have spare pumps and stones which came in handy last winter when we hit 0' and the pump went out.. I just hooked the second one up and was able to keep the hole in the ice.