sps problems

ok guys 2 weeks a go i started to notice a couple of pieces of sps started to bleach from the base up its just turning white and not flakeing off its been progressing verry slowly on most of them so first thing i did was a search for pest/parisites on them but found nothing.i am going to give you the prams of the tank next
tank is 75g
ph flux from 8.0 to 8.2
dkh 7.7
alk 2.74 meq/l
cal 440
mag 1300
phos 0
nitrite 0
nitrate 0
ammonia <0.02
temp 79.4 to 81.6
i am going to provide as much info as possible but if something is missing or smething else needs to be known please let me know. For flow there is about 2000gph lights 2 150 halides 14k and 2 48 t5 overdriven ,cal reactor ,phos reactor,phos reactor with carbon,tank is skimmed and filtered. now the other tank mates for coral are xenia,kenya tree,star pollyps,duncan and some zoos + some other various pieces of sps. all pieces that are bleaching are the newest to tank and have been in for about 2 months. i will put a pic of the worst one anyone please let me know if something is missing and help would be much appreciated thanks

 

How long per day do you keep your lights on?

 

Well, usually if it starts at the base, and progresses up, that is RTN. But that happens VERY quickly, which you didn't describe.
Can you get a pic of it? The only thing I see in that pic or whitish tips, which is probably just growth.

 

t-5 on for 10 hrs mh on for 5 and i will try to get a better pic in a min if u look at the plug its on the part that is half white was coloured at one point

 

Seems like a good amount of lighting times if your temp remains that low throughout.
Like packleader stated, I see that only the frag plug is turning white. This could mean that only the Coraline is bleaching/dying and not the SPS.

 

heres a better pic

 

You might want to try raising the pH up to 8.3 Not sure if low pH would cause this... but you never know.

 

How long exactly did it take it to recede that far?

 

That happened to two of my SPS when my alkalinity fluctuated a lot. I fixed my alk and it fixed itself in a few days time. Have you doublechecked your params against other test kits or at an LFS? Also, what is your salinity and has it been doublechecked against a calibrated refractometer?

 

Your water parameters are off for good SPS health and more then likely the flows are not adequate for them either ? and any sediment gathering at the base will cause that also

Now DKH and Alk mean the same thing and they are both way to low the Ca is good but once you raise the DKH to proper levels and get that Magnesium up the ca. will fall off to about 400 or less ? Keep the Ca at 400 Minimum .

dkh 7.7 You want this at 10 at least or 13 is better I keep mine near to 14.
alk 2.74 meq/l This would be 500 at 10 DKH
cal 440 This is good but will drop off when you correct the DKH
mag 1300 you want to bump this up to 1450 to 1500 ppm

Oh forgot take the corals and iodine dip them asap and snap off a few healthy tips to frag them just in-case white band disease can sometimes be totally stopped with iodine and or an antibiotic dip I use red slime remover for the antibiotic dipping