Tragedy Befalls the Tank

So far, everything has been going quite well for my little tank, that is, until the other night. It's been rather warm lately, but I usually keep a close eye on the temperature of the tank. Most aquatic insects have no problem with cold water, but water above 75 F can be deadly. Part of this is due to their inability to withstand the heat and the fact that oxygen becomes less soluble in warm water and thus oxygen levels will drop. It had been floating near 80 F for the past couple of nights, which created quite a few emergence events. This lulled me into a false sense of security, so on a night I was otherwise occupied, I assumed that everyone would be fine even if the tank heated up.

A tusked mayfly and its subimago stage (winged juvenile).

Subimago to the left and adult on the right.

I was quite wrong. I came upstairs to grab a few things and found that some of the mayflies were out from under their rocks, one in fact had lodged itself against my divider. I took several pictures then returned to the party downstairs, not realizing how odd that was.

Tusked mayfly.

When I turned in for the night (3am ish), I found all of my mayflies out on the rocks. Their gills, located along their abdomen, were flapping frantically. The temperature of the water was above 86 F. I did not think this directly related to their odd activity, in fact, I simply thought I was observing a new behavior due to it being to late in the evening. Why wouldn't aquatic larvae be nocturnal, most of their predators are diurnal anyway?
When I awoke the next day, all of the mayflies were dead. It was quite upsetting and made me realize that I must find a way to maintian the temperature of the tank in the 70's at the least. I figure that removing the lid and just having mosquito netting over the top will naturally allow more evaporation and thus removal of hot water from the system. If this does not work, I may have to research some type of tank equipment that would do the job.