How much do you feed the tank? How many fish do you have?
I ran into this problem with my tank (BB, no fish, no additional feeding, etc)....basically it became too sterile...and SPS began to decline. I've been heavily feeding for ~3 weeks now and don't see anymore lightening or necrosis....and might, actually be seeing some improvements *crosses fingers*
Remember, photosynthesis can't make something from nothing (first law of thermodynamics)! Zooxanthellae need a nitrogen source to complete the whole process........and the coral polyps provide this by prey, absorption, etc.
EDIT: I've read that "rusted screws" thread several times...and everytime, all I can do is this about all the people running "x" number of GRAMS of GFO in their phosban reactors. 6 screws with surface rust are the issue, I wonder if this pumps have other "issues" like a cracked housing or something.