FYI - I just built one of these this year and used it for the first time in August.
I am using just plain tap-water in the cooler/tank and was able to get it down to 34F with no freezing of the water....or none that I saw anyway. I use a very small wave/pump to circulate the water in the cooler/tank.
I also only needed to keep the cooler/tank 4-5 degrees cooler than my set temp in the fermenter. If I had a bigger gap than that, there was a larger fluctuation of fermenter temps. When I kept it closer like that, it kept a pretty consistent .25 degree each side of the set temp.
I did cold-crash the Belgian Tripel I just made -- well...cold crash may be a little generous. I dropped the temp to 37F in the fermenter for 3 days. The temp stayed rock-solid while the cooler/tank stayed at 33-34F.
I suggest using water a few times first if your air conditioner is used. Mine was VERY used and VERY dirty. I cleaned the coil the best I could with a brush and liquid, but after a few test runs, the water still gets pretty dirty (and stinky). I'm hoping that soon the coils/fins will be clean and won't dirty up the water any longer. Only after the water stopped getting dirty would I use Glycol.
I'm using a Raspberry Pi and CraftbeerPi software.
This is the fermenter temp:
And a different time/date of my cooler/tank temps (not for the same brew as the graph above):