I have guppies in one of mine, and with their tendency towards ich (female guppies, when preggers are prone to being chilled, and thus contracting ich) I run mine a little hot, usually 80-82.
My aggressive tank is kept lower, about 76-78.
But on average, any were from 76-80 is fine. But you have to give a little leeway, if it's 75 or 82, they won't die. It's just best to have it around 76-80.
Best of Luck!