Another fairly inexpensive idea is to build a peltier cooling system. I have some parts on order to build one because I have the same water temp problem (but no cistern, LoL).
Using a couple cpu fans with heatsinks, you can build a "cold air" system, but for water cooling I am replacing the cold-side fan with a heatsink, to which I will add some copper "rods or tubing" to extend into the reservoir water and hopefully transmit enough cold via thermal conduction to keep the water temp down.
Another alternative is to use a waterblock approach, and
pump water through it with another pump, or if big enough, T-off the existing main
nutrient line with a recirculating line going through the waterblock on the peltier unit.
The trick is of course, to figure out how to transfer the cold temp of the cold side of the peltier into the water, and there are a lot of ways to do that. Check out youtube videos galore for peltier ideas. If you DIY, they can be very inexpensive cooling/heating setups.
Given all the variables, I just took a WAG at the size (watts) of the peltier unit. I will do some testing in various sizes of large camping coolers to see how much differential cooling I can come up with.
MJ