From all the reading I have done, here is how I break it down. Cooler res. temps allow more oxygen to dissolve, reduce the chances of many different types of slimes, fungus and general nasty smells. It can get to a point where it is too cold, say below 60 and it can cause a bit of a slowdown in growth and have a negative effect on the good kind of microbes you want to have.
I have a 1/2" stainless coil in my reservior that I pump glycol water through from a res. in the freezer. I have let it run for 2 hours straight and the res. was in the upper 40's when I finally stopped ( I think that is just about as cold as it would get with the setup as it is now) which would more than likely kill whatever I dumped that on...lol. I will get a chiller at sometime, but not for 1 res and 4 plants at a time...lol
Peace and Great Grows
Asmallvoice