Looks to me like heat stress and overwatering, (watering too often) and I'll bet they are ready for some half-strength nutes. Raising the lights an inch or two wouldn't hurt.

Might want to do something about that fire hazard, too. GFCI's are not to be trusted THAT much. Great for power spikes...but water and electricity can cause fires, (not necessarily power spikes) which the GFCI doesn't protect against. :thumbsup:

I second the 'ditch the probe' sentiments. I've never heard of one actually working properly for cannabis.