I have a new idea:
If you have a sump, you can build an a "chiller" chamber next to the return chamber, but instead of using glass to separate the chambers, a material that would allow temperature changes to dissolve through (ex. coldness from ice moving from inside a ziplock bag onto your skin). A chilling mechanism (freezing metal coils) would be left in the "chiller" chamber to decrease the temperature of the water in this chamber. This will allow the transfer of cold temperature from the metal coils into your SW
, but will not run the risk of contaminating your water. Rough drawing below.
Blue = SW
greenish water = chilling water
black box = chilling machine
brown = metal coils
dang, I didnt think the controller box was that much!