Oh no...I'm sorry to hear about the firefish
That stinks.
Summer is always a challenge with the temperatures and evaporation etc. I have closed tops on my tanks...but that is because if I didn't I'd lose more water than I already do. During summer even if people dont use chillers you have to figure out how to keep the temperature somewhat steady. Some people use the open top and a fan going across the surface...
I would think that the closed top does keep in some heat...but then again with fish like the firefish you can't have the top open. You could use that eggcrate stuff (like a metal mesh that you can cut to the tank size so that no critters can get out, but there will be gas exchange)...see if that makes any difference.
I have AC, so luckily my house rarely gets up that high even in the middle of summer. A lot of fish can handle the really high temps (esp if they dont go up and down very quickly...but i'm guessing since they were pretty new to the tank they were probably a little more susceptible to getting stressed out/shocked...