Do you have a cold water source near by? Some of the reef guys that have big tanks with sumps use a coil made out of pex (flexible plastic) pipe in the sump to heat the aquarium and they use an electronic valve controlled by a thermostat. Maybe you could rig up something similar but with cold water to cool instead of heat. I don't think it would have to be that complicated. You could have a simple heat ex changer made from a piece of PVC pipe with one of the hoses from the canister filter running through the pipe, and you could vary the length depending on how much you would need to cool the tank water. Does that make sense?