I have a single metal halide bulb fixture. I changed from an old 150W 20k bulb to a new 150W 20k bulb.
This problem occurs when you don't acclimate your corals to a significant change in light intensity. There are a number of ways to do this, including lowering or shading the corals or raising the light fixture. As far as I can tell, it's a pretty common mistake for inexperienced reefers (like me) to make.
My question was not phrased well, but what I am really trying to understand is whether there is some way to treat the symptom of zooxanthellae expulsion.
Not from the lights ! try to find another reason ,,,
Keep zooxanthellae well fed.
I agree with your diagnosis mokusei. I just recently changed out my ballast and did not realize the lighting difference. I also have a few that are showing signs of bleaching and have shortened my photo period and just time will tell whether things turn aroundOnly the high placed corals in my aquarium were effected. Were your bulbs rather old that you replaced?
The bulb I replaced was 6+ months old - not sure exactly how old because it came with the fixture which I bought used.