LOL!!! I've NEVER run 18/6.
By your logic, I should NEVER have had any plants grow!

Here's a good rule to follow:
If you are convinced that your plant is in shock from transplant, root trimming, root rot, etc., MAINTAIN your previous light schedule and DECREASE the intensity of the light. The thrid thing I do is do a LIGHT foliar mist of an anti-transpirant- lately I've been using SMC leaf shine.

The point is, photoperiod changes add stress. So don't change that. But if your plant is transpiring water faster than the roots can replace it, you're in trouble, so slow the plant down by raising the lights.

I have a 'backup' t5 fixture in my grow room so if I have a problem I can just kill the HIDs and run flourescent until the danger has passed.