I must step in with my own opinions here. I am not a fish biologist but have over 50 years of experience raising guppies. My best results, in terms of health, have always been at lower temperatures. I cannot offer any scientific evidence that one temperature is better than another for them, but I can say that the slower growth and longer fry maturity associated with lower temperatures seems to give me superior fish. My fish never experience any disease problems and are always treated by other fish keepers as if they were in some way superior to other fish. My fish are raised on simple flake food, something many fish keepers frown on, and are still seen as superior by most fish keepers. The only thing that I know that I do different to many fish keepers is to refuse to use high temperatures. As C101 has said, there is no truly scientific evidence of the superiority of my method, but I remain convinced that the one thing my own fish have, as descendants of fish from my LFS, is that I refuse to raise them at a temperature that accelerates their growth rate. I simply never see any disease problems in my guppy tank and the fish I offer for sale at auction, simple generic "fancy" guppies, are always snapped up by other fish enthusiasts as if there were something special about them. Sorry folks, those fish are no better, from a genetic standpoint, than most other fish at the LFS. Their condition simply reflects that I am an old man that has seen lots of problems over the years trying to keep guppies at higher temperatures where, I am the first to admit it, the fish seem better on average on a day-to-day basis. The problem I see, from the point of view of a long term fish keeper, is that the fast track short life cycle of fish kept too warm is anything but ideal. Patience is a seldom admired quality but is essential if you want superior quality fish in the longer term.