bro lemon,
i also using solely NSW for water change every week, possibly from the same source as u..
while i was home 2 days ago, everything still seemed normal to me.. (staying in school hostel now, preparing for exams) but when i get home tonight i will go take a closer inspection on my frags.. and update here again if i notice any anomalies..
i do agree, sometimes things turn funny and we might not know the cause of it.. the feelings ain't good.. usually for the case of SPS, whenever i notice something awry, i frag it, and place it in another location or tank (usually my dad's tank), and hope that at least i get some 'seeds' to survive.. and not lose it all.. very rarely will we encounter entire tank corals wipe-out, which i hope its not in your case..
if like what cedric said, sometimes its just THAT particular piece that is problematic after shipment and decides to die on u after some time, den theres nothing much we can do also..
anyway, all the best for the rest of your sps, i'm sure they'll turn out fine.. once i get home, i update u again..