I wouldn't be overly concerned until the tank water starts pushing past about 84 degrees, then you can try freezing 16oz water bottles and floating them in the tank.
A fan blowing across the surface of the water will also provide some evaporative cooling as well.
Be sure to maintain good oxygenation of the water as the temps climb, either by dropping some airstones in there or some other means of good surface agitation that will be sure to circulate the lower layers of water to the top continuously.