I was running 2 lights on my 29 gallon, a 36 watt coralife t5no and a 20 watt t8 tube, just to see what the effects would be. I normally just run the T5NO. So I decided to turn off the t8 and since I did that I have what appears to be thread algae. Why would less light trigger algae?
A sudden change in conditions perhaps let algae take foot in your tank. Or it could have been that the algae problem was already nascent and you turned off the light just as the problem began to occur. Increase the light again and see if it stops the growth of the thread algae.