I had 2 go as well 3 weeks ago.. :eek:

Re RTN
 

I lost a few SPS frags in the last few weeks as well.
Interesting that there seems to be a trend here.
I personally have no idea why they died. Others in the same tank are doing fine and a couple that started to RTN are now slowing coming back.
Weird.
Just goes to show you how truly little we know..