Water thermostat/aquastat

I need a thermostat that will control a pump. I want to build a recirculating cooler by running copper tubing in the my ventilation intake.

Water will be run through a coil immersed in the nutrient solution and then up through a coil in the ducting and back down. During the summer, I won't have an issue with it running, but in the winter if I have this running to much the nutrient solution will be ice cold. I need a device to turn the pump on and off at my desired temp.

Any ideas?

On ebay they have "temperature controllers", most of them from china. They are made for aquariums, and I believe they work in the air as well. You can get them in 110v or 220v. They also the kind that turn on when too hot, too cold, or ones that are capable of doing both (toggle option like a normal thermostat). I posted a link somewhere in DIY. It was "DIY temperature controller". I think the amperage limit on these might be low, so you would have to connect it to a relay for anything but a solenoid or something.

Not exactly sure the setup you have in mind, because I've never done anything like what you are describing, but maybe something as simple as an aquarium heater. They have built in thermostats.