Temperature.

I use a neptune systems aquacontroller III. Its probably overkill for what your looking for, but its an option so I'll share it. It allows me to control my lights, powerheads, pumps, heaters, chiller, top offs, monitors ph, orp, and temperature, plugs into the computer, and has a feeding cycle which is neat with alarm outputs to signal you in case of catastrophic events. I love it, but it is spendy, around $400.00 I don't know for sure because I traded my old aquacontroller II for this new one.