My suggestion would require quite a bit of water but if you could do a closed loop system and it would be much cheaper. Let me explain! A chiller is nothing but a heat exchanger using a chemical. I recently have made a copper heat exchanger for my father using copper pipe. It is used to cool beer during home brew. It has a garden hose fitting on either end. The water coming into you house is always cool due to how deep it is in the ground. You could use city water or well and run the garden hose threw the pipe in your sump and you now have a heat exchanger. The water then runs out of the pipe and to drain. There are also set ups that run a recirc line to a cooler filled with ice. Using a return pump you then send the cooled water back to the coils in the sump. I know we dont wanna use copper in an aquarium but this could be done using 1/2" poly tubing or stainless tubing from home depot. If you are really handy you could figure out how to set this up to a thermostat and a solenoid valve and you would pretty much have a chiller. This could also be used as an emergency fix for someone who has lost there AC or even power to there house and the ability to keep the tank cool.