Homemade chiller for nutrient? An idea

[cardinal]

I have been reading about the importance of keeping my reservoir from getting too warm, in order to avoid disease. Coming from a cold place, it has never been an issue to me before. Cold basements seem to have sorted it out for me.
Now I live somewhere hot, and the ambient air temperature is above 90 degrees fahrenheit, 32 celcius for weeks on end. Anyway, I was a bit taken by surprise at the prices of chillers that the aquarium guys sell.
What do you think about me using a normal fridge holding a vat of cold water, and running my nutrient solution through tubes into it, around and around inside it, then back into the reservoir? I'd just poke a hole in the fridge door, and run the 2 tubes through, then fill around them with expanding foam.
I think this would work.
Now... if I'm KEEPING the temp down to 68 F, won't that be a bit of a shock for the old root zone every few hours? (flood and drain) - I thought we were supposed to be gentle with them.
I'll be air conditioning the atmosphere to 89 F, and injecting CO2.
Any comments from the learned would be appreciated.