Sure. Just goto your local hardware store and get a thermostat. It will take a little electrical work, but in 30 mintues or so you are set to go. There are also temperature controllers you can buy (check various online reef stores) that heaters plug into and are then regulated by. Some require special heaters though, so do some research. Hope this helps!!
I installed a temperature controller that was very precise. (Can't remember the manufacturer/model, but it was designed for controlling temperatures for brewing beer.) While it worked, it was amazingly precise. However, it failed (or the sensor failed) in about 3 months and the temperature skyrocketed.
This may be a case of keeping it simple is best. What I do, however, is run two heaters in parallel, set to the same temperature, each capable of heating the whole system. If one fails, I am still in good shape. It is far more likely that a heating element will burn out, than the theromostat running amuck.
Yes I understand that they do fail to the off side but my guess is they can easily fail to the constant on side. I am always repairing electrical relays and other electrical switches that have fused shut and thereby leaving whatever equipment they are attached to in a constant on position.
I also run more than one heater in my tank for the reverse scenario but would like to cover my butt, and keep from making fish stew
