84-85 degrees betta tank?

84-85 for short periods of time is okay. However if it's going to be more than a week or two then you should do something about it. A small desk top fan pointed at the tank should work for a small five gallon tank. (I'm having the same issue right now.) Turn off the light, lower the water level an inch or so so that the filter splashes more. Keep your heater turned to 78-80 though to keep the fan from making the tank too cold. Honestly though I don't think it'll be much of an issue. A lot of them come from Thailand and the water temperature there is quite high as well. As long as it's coming down to 78-80 at night I wouldn't worry too much. Keep the light off though. That will help with the heat.