Ok, pretty high here so bear with me....
Your idea got me thinking. You could take a length of copper tubing and coil it a few times, being careful not to crimp it. You end up with something similar to a wort-chiller used in homebrewing. Drill 2 holes in top (or side or whatever) of mini-fridge to line up with the 2 ends of your coil. Fit the coil in place and seal it up with an assload of silicone caulk. Now connect a water pump to one end of the coil using tubing & run tubing from the other end of the coil back to the res. Place the pump in your res & it should start circulating water through the now cool coil and back into your res. If you ran the return line back higher than the water level of the res it could also help aerate the solution from the waterfall effect. You could control temps using the thermostat in the mini-fridge.
Seems like a pretty cheap alternative to a res cooler for anyone with a mini-fridge sitting around and a little time. Or maybe I'm just stoned.
