Coral Bleaching

So, I have a 5 year old 65 gallon that I came to realize had "old tank syndrome." The very deep (6 inch) sand bed had become ridden with sulfur. Even though I was running GFO 24/7, I couldn't get the hair algae under control. I decided to break it down, clean it and put a new sand bed in. But, before I did this, I wanted to save my corals (lps) and livestock, so I took a 40 gallon tank, added fresh saltwater, a hob skimmer, powerhead and a new LED light, so the coral would survive the rebuild. I acclimated the coral to the new tank for about 45 minutes and released them into the new tank. I set my lights to come on for 6 hours a day in the new tank. After a day or two, I noticed that most of my coral started bleaching! Salinity is 1.022, Alk is 9.688 temp was originally a bit low at 69 degrees, but I added a heater and now the temp is maintaining at 74. Phosphate is .05-.10.
The only difference between the 65 gallon and the 40 gallon is the temp (68/72).
What else could it possibly be? Perhaps the intensity of the LED?
Thanks