It would be geothermal cooling in a sense.
What is the average temp of the cistern? I'm sure there is a scientific way of figuring out how much coil you would need to chill the nute tank x numbers of degrees.
Depending on how many gallons your nute tank you might want to test a few things out. Drop a small
pump into the cistern and run the water through a coil of say 25-50' feet of pipe and see how it works out. I say small pump because there is no need to move the water that fast. It can only exchange heat at a slow rate because there most likely is not a large difference in the water/nute temperature. Your cistern is what 65-70 degrees not 40.