Personally I've not had problems even outdoors in hot Cali sun with water temps past 90 degrees as long as the reservoir is large enough, has a TON of oxygen being pumped through it, and has great circulation so it doesn't go stangant.

Also, that added motion helps with the cooling process. Convection helps a tiny bit.

If you really want it cool, get a Peltier cooling module, fix a thin heatsink and fan to it, pop it on the side of your reservoir, hook up power. This should help immensely. For a small 5 gallon reservoir, you'll only need a 60 watt one to help bring that temp down.

The majority of root rot is caused by a lack of oxygen and circulation. Pathogens that cause this thrive in poor reservoir conditions, even at room temperature.