Yes, there are controllers that read the temp of the tank and are designed to turn on heaters and chillers, depending on what your settings are asking for.. You can have them turn the heaters and fans on and off, instead of a chiller....
The cost in in the area of $120 to $175.00...
I have been thinking of doing the same thing, because I am having a little trouble getting my temp as stable now that I have added the metal halides.... Instead of a constant 82 degrees, I have been fluctuating from 81.5 to 82.7....
Not sure yet if I am anxious to spend that much over that sort of variation in temp..
Dave