temp monitor for lights

[Samw]

Diomedes. Thanks for the suggestion. But I already have the fan turned on throughout the day and last year, it still hit over 30C with the MH lights on. This is why I need to control the lights rather than the fan as I can leave the fan on all the time.

Buk_A_neer. This is a good idea but I assume the temp sensor can't go underwater and must need to be plugged into an electrical outlet. Is that right? There might be a difference though between the room temp and the tank temp. Hmmm.

I haven't finished looking at the Ranco devices yet. It might be what I want. Not sure yet though if they can be programmed to turn a device on when a low temp is reached and turn the device off when a high temp is reached.