New to SPS -- Skin falling off one piece

[ps2cho]

So I kinda lost as to why one of my SPS frags is having necrosis. It happened probably a few days ago.

Salinity: 1.025
Temp: 79c
dKH: 6.5
pH: 7.8
Nitrate: 1
Calcium: 400
Phosphates: 0.1

My Mg keeps reading 1040-1080, but that is even with a fresh batch of salt, so I am not even sure if the test kit is right or not.

What should I do here to stop this happening?
I only have 2 small frags in a 29g tank. I do 17% water changes weekly with Red Sea Coral Pro salt.

 

[ebond009]

Did the dkh lower dramatically recently or has it always been on the low side? How old are the frags? I would look into sudden changes that could of happened.if it started on top cut off affected piece .ive also read that you can apply super glue on base of the rtn and sometimes it stops it.never tried that though.GL.

 

[PaleHorse]

Get another test kit/bring water to LFS

 

[LoJack]

Alkalinity is really low. You need to find out like ebond says, if it's consistently low or just had a recent drop.

Alk fluctuations will give you more grief than almost any other parameter when it comes to SPS. You need stability.