Absolutely there is evidence not only to indicate, but to prove that fish stress when the water temp is increased. It's one of the tests they use when they want to create stress in fish for studies. They call it a temperature challenge. They have measured the stress hormones in the blood when fish are subjected to increased temperatures... or decreased for that matter. Besides stress. increasing the temperature allows for less oxygen levels in the water. These fish have more Ich in their gills than you can see on the outside. Ich LOVES the gills because of the easy access to the fish's blood supply. When the gill filaments are infested with parasites, the fish cannot oxygenate as easily. Now you've added another stressor besides just impacting them negatively physiologically. Ich does not love cold water. They are pathogenic in water ranging from 68-86. Optimum reproductive temperature is 86. Most do not complete their life cycles lower than 66 degrees F and stay in the encystment phase until the temperatures are again risen. You increase the temp and you are going to have higher numbers in your tank to feed on your fish. The faster they go through their cycle, the more offspring they produce. Every time one single parasite completes it cycle, they on average produces between 200-300 theronts. So as the temp goes up and up to a peak of 86, they are able to turn out more and more and more little vampires. The numbers above are the majority, NOT the absolute. There have been Ich found and reproduced under some very varied conditions.