I have had a recent experience with what I think is chronically low calcium that caused bleaching and tissue loss.
I normally run my SPS tank with the following parameters
Alk= 7.5 -7.8,
P04= 0
Nitrates=0 ,
calcium=400.
Over the last month or two, my acros started to get really pale, some of my stags even bleached in high light spots and some begun to lose tissue at the base and undersides.
I had made no changes at all in months.
At a loss, I got all new test kits (or reagents)
I found that my calcium is actually down around 370-380. While that in itself is not that bad, I think its low enough that when pushing every other parameter to the minimal, it was the final straw.
within just a week or so of bumping the calcium up to about 420 things are starting to look better.