Your water chemistry is definitely off to be growing SPS. In particular:
Nitrite (NO2)Good=0.005 ; as mentioned above you should not have any measurable Nitrite in an 'established' tank. Look for undigested food in your sand bed, dead fish behind your live rock, etc.
Phosphate (PO4)Good=0.01...
+1 on Alk/Phosphate being the most important (IMHO). Ca/Magnesium really don't vary that much unless you have insane SPS/clam/LPS growth. Alk on the other hand gets burned up real quick just from normal biological action in your tank, and you know about Phosphate...
Magnesium really isn't...
Not a big deal. Just don't make any drastic changes to your tank and you'll be fine. You can start by doing your normal water changes using the new salt and slowly transition over. In reality, alk and phosphate are the only two parms I normally routinely check. Most other values tend to be...
Before adding GFO etc would could mask a problem find out the source of your measured 6ppm Phosphate level. Most (low) phosphate test kits max at 3ppm. Which test kit are you using and are you sure it is 6ppm - as that is really high. What is the measured phosphate using your existing test kit...