The issue with higher temperature is not just the temperature level exactly but the lower amount of dissolved oxygen content in the water.
Oxygen by the way is the driving power behind a healthily functioning tank. Fish that come from cooler waters are not adapted to living in waters with low oxygen content and even those that do, would still benefit from a tank richer in oxygen, although a lot of fish keepers are oblivious to the effect of low oxygen levels in a tank. It wreaks havoc on the entire set up long term, from nitrification effectiveness to the type of micro-fauna establishing, to direct impact on fish health.
Oxygen is difficult to dissolve in water.
"it was culculated that it would take 6 years for oxygen to diffuse from surface to a depth of 6 meters in quiet water. "
http://www.fao.org/docrep/field/003/AC183E/AC183E04.htm
And anything can tip the levels, especially non-planted tanks with higher organic levels either due to overfeeding, overstocking or simply because they've got rich fancy substrate going through heavy decomposition....
Anything that is exerting great energy just to "breathe" would have a shorter lifespan....Fish that come from habitats with higher temperature, and are adapted to lower oxygen levels, are still not immune because in their natural habitats oxygen is not an issue but it is in aquariums...Take for example a heavily fed discus tank kept in the high 80s or 90....a recipe for disaster I think....It is certainly very difficult to maintain a healthy environment in such a tank. People think they siphoned the tank and did the water change, hence water is pristine but the organics get dissolved in the water column, you can't see them, so you can't ever completely "clean up" a beef heart fed tank unless you drain the water....the effect will be present....and in many cases disastrous...
What is a typical fry tank or a discus tank? Sponge filters, low flow, low surface agitation, high amounts of food, high temperatures...A recipe for oxygen deprivation....Blame the pH then....or fish sensitivity., bad luck...or even lack of adequate water changes...but you can't replace oxygen via water changes, you can just mitigate the effect and prolong the battle...