Problem Lightening corals (bleaching

[Ike]

Sure MH UV is a good way to kill corals, but that's why you run them behind UV shielding glass. There is no aquarium light when run properly that you can switch on and off again (or leave on for an hour or so) that will bleach corals.
I myself run 2x 400W MH moguls + 4x 54W T5 on a 100 gallon tank so have experienced in the past what happens when you change really, really old MH bulbs for new bulbs without adjusting photoperiod.

For me this is a classic case where nutrients were low, total light input was increased which increases demand for nutrients that weren't available. Evidenced by bleaching from the base being worse to keep the growtips relatively healthy.
But because changes happened too fast as well the corals are now actually starving so even the growtips show signs of bleaching.
If it had been light induced the growtips would be bleaching the worst and the base be looking relatively healthy (so survival would depend on the base spreading out in the future).

I mostly agree. However, a full bank of overdriven T5's are absolutely capable of bleaching corals faster than any aquarium light I've used (most combos of T5 and MH)... Most corals could handle it for 6 hours or so, some looked better with more, and others I contended at the time, and still believe that T5's are tough on certain corals due to their blanketing corals in an unnatural way. Whether you want to argue if overdriven is properly run or not is up to you

I also think you're mixing up bleaching vs. lightening, but a lot of people call lightening bleaching... A bleaching event wouldn't be caused by typical aquarium lights, but lightening of corals certainly can, even after they're well acclimated.