With this Setosa, the whitening started at the base and did not have polyps in the dead areas. I know my pictures are not the best, I use an iPhone camera for them. The area was definately in my opinion dead, and slowly over several weeks the dead area grew a little bit. I noticed a whitening of the edge of new growth which slowly coloured back up.
I have been very cautiously and slowly raising my lighting and started below the manufacturers recommendation.
I got to 51% going 1% per day and the issue started atleast a week or two prior to that point.
I noticed it when I lowered the setosa in the tank after the switch to LEDs, so it likely was still in the 30's percentage wise if I am not mistaken.
I am obviously new to SPS corals and have taken the approach that I won't learn unless I do. In my opinion this issue was STN.. What the cause was, I don't know.
My lengthy dip in a heated container with an old pump for circulation was obviously too strong/ long and likely has killed it.
My cyphastrea, chalice and my acros and montis which came after the issue arose, all look great and healthy.
My mag and alk did drop to the low end of the scale the week or two leading up to the issue being discovered. I slowly raised them back up with water changes and B Ionic dosing.
I really don't know what happened to this coral, I can guess and likely be wrong. I have read and researched my brains out and other people have been in the same boat as me, with no definitive answer as to what happened, but everything else seems fine in the tank.
It is a newer set up. Very young for SPS coral but half of the rock was live rock from a long time established SPS system.
I've learned a lot dealing with this issue, importance of testing weekly atleast, keeping mag up so alk doesn't bottom out, and hopefully I can keep everything moving in the right direction.
Hope this thread helps someone else one day..