First of all, wouldn't copper tubing be way more efficient for heat exchange? Secondly, have you thought about using a wort chiller for brewing? Hear me out... Instead of running your nute solution through tubing encased in a block of ice inside the freezer, what if you instead had a reservoir of antifreeze in your freezer, below freezing, and you pumped that through tubing into a copper coil inside your control bucket. That way, you have the coil in the control, and when the temp gets too high, the pump kicks on and circulates antifreeze that is below freezing in copper coils in your control, instead of pumping your nute solution through the chest fereezer.