It should be remembered that there is a direct relationship between temp and Dissolved oxygen (DO). At lower temps there is more oxygen than when the temp is higher. Stream / creek / river fishes generally require higher oxygen intake than lets say a lake type fish. Raising the flow and agitation of the surface water will increase DO content in warmer water.
What you are seeing when you see the fish happier in colder water than in warm is often it is receiving more oxygen in the cold water than it is in the warmer water. Species is really a necessity to know so that one knows the lethal temp of the fish as well as the level of oxygen it will require. One can keep many "coldwater" fish in a warmer temp tank as long as this relationship with temp, oxygen, and flow / agitation is understood.