75 to 82 is optimal.. so 80 would be fine and if it dropped to 75 that'd be fine too, i mean these frogs live outdoors so obviously at night time is going to drop the temperature in their natural habitat. doesnt sound like you need a heater.. save it for winter..

You are sure that your frogs are african dwarf frogs and no african clawed frogs right? Clawed frogs need much lower water temperature.