Honestly, I've never seen a heat death myself so I'm not sure what they look like, but that would be my first guess. I am so sorry for your loss. Although there is ice in the water, even more important is that they have places they can get out of the heat. For example, a place in the shade where they can dig into the ground and wait out the hottest part of the day. And, because the sun moves and therefore different areas will be hotter at different times of the day, they really need multiple locations so they can choose the best place to be at any given time. Our temps are been around 110 the past few days (111 at one point today) and I haven't put ice in the water, but because they have plenty of space to move around and get out of the heat, none has looked particularly heat stressed yet.
Several people I know have bought misters to set up and keep a certain area cool. I guess the misters make about a 20-degree difference in temperature. Is setting up something like a possibility to prevent more losses?