Looking for cooling fan temperature switch

[ZoeReef]

I'm looking for a simple solution to turn a cooling fan on for a 25g nano. Temp got up to 83 so I set up a small cooling fan. Temp dropped this morning to 74. Obviously too big a swing.

I'm hoping there is a radio shack gizmo or similar that would sense water temp and switch the fan on.

Anyone got an idea? Thanks

 

[bertoni]

Lots of places sell temperature controllers. A two-stage will turn on the heater when the tank gets cool and then turn on the fans when the tank gets hot.