Soon after starting a saltwater tank I began keeping seahorses, and common pathogens of seahorses are much less virolent at 74 degrees and under. When I saw that there was no harm to the corals that I was keeping (gorgonians, zoanthids, 'shrooms, LPS) or other fish, I simply began keeping my other tanks without heaters; heaters are such a liability anyway. Right now I do not have any horses, but I don't see any reason to change the temperature of the tanks.
The only detrimental effect that I ever saw was when one tank hit 64 degrees for about a month -- xenia did not appreciate that (but neither did some hair algae, it totally disappeared.)
