Need to know why you have a ten degree swing in temperture. If it is from your lights you can mount a small cooling fan to blow across top of tank when lights are on. This should keep temperture constant when lights are on.If you have a power head on it might be causing to much heat and might have to go to a smaller one.
I have both light and powerhead I have not seen the light on for the heater so not even sure if it still works it is about 10+ months old but less then a year. Not sure what it is. I live in south Fl so it stays warm here.
There are chillers just comming out that are designed for smaller units. Browse the following at least: Marine depot, Dr.s Foster & Smith, Premium Aquatics, That fish place, hope this helps.