I assume it isn't in the sun or anything? Does it even have a heater on it, or does it just stay warm? As long as it stays warm and does not fluctuate during the day, your fish should adapt to it. If there are fluctuations, and your heater isn't on, turn your heater up to 82, and just keep it at that temp. Its a little warm, but its better than "up and down, up and down". I think a chiller would be an exorbitant amount of money and not really worth it also...
what kind of lights are you using? anything that gets hot? like incandescents?