Backyard Aquaponics
A place to discuss aquaponics

A diy chiller with a refrigerator would be possible using a closed loop and inline pump with a temperature probe connected to controller for the fridge and pump. You could put a copper coil in the fridge, connected to a fish safe coil to go in the tank, then recirculate the water with the pump.

Donald Porta did a similar idea with a hot water heater to heat his tilapia tanks in his greenhouse.
https://www.youtube.com/watch?v=_YOk7UOKXi8

A commercial available aquarium chiller would also work, though it would have to be sized for temperature and system size, so would probably be a more expensive, though easier option.
One found on a random google search for aquarium chiller
http://www.aquaticwarehouse.com/chiller ... oCO3vw_wcB