acropora bleaching

i have had this acropora in my reef for about 9 months. This weekend, the temperature of the reef dropped about 3 degrees probably rapidly enough to damage this coral. none of my other corals seem affected by it. I changed my metal halide bulbs about 3 months ago, so i doubt deteriorating light quality is the culprit.
About 1/2 inch of the tips are bleached, and it seems like i'ts getting worse. The only thing i could think of to do was break it up into 9 frags so that a few may survive. I hope i didn't jump the gun on this. Any advice on what else can be done would be greatly appriciated.
I did buy another heater and will be sure to keep the temp in the room stable.
I do have other acropora spp. that were not effected, so possibly this temperature drop wasn't the problem??
Thanks in advance for any suggestions
 
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