Help! What killed my Coral?

Last night I decided to start growing coral in my tank, I bought a rainbow ricordea, pulsing xenia, and a small kenya tree. I properly attached them to my rock and they seemed to be doing good. They survived the night and by morning they were starting to open up, however when I got hom from work today the Ricordea is gone (im assuming eatin) and the xenia was missing its stem and just laying on the rock. The kenya wasnt touched. Everything is my tank is reef safe, so i thought...... Here is a list of the live stock in my 40 gallon tank (about two months old)

1 Blue yellow tail Damesel
1Yellow Damsel
2 Blue green Chromis
1 Lawnmower blenny
1 sand sifting star fish
5 Hermit crabs
2 Scarlet hermit crabs
5 Astrea snails
2 Turbo snails

Any help would be appricated, I really want corals but I need to eliminate the problems first. thanks

 

judging by the livestock, it could have been the hermit crabs. but, i'm just curious, what are your parameters?

 

I have noticed that Damsels will sometimes move things off "their" LR when they do not like it there. I had a damsel that use to move the frag plugs if I did not glue them down.

That being said, nothing you listed are "coral eaters" so to say. Your hermits may eat coral, but only if it is dying already.

How did you attach the corals? Did they come on plugs or did you glue them? It can be something as simple as the damsels moving the ricordea because it was not secured and the xenia being retracted because of acclimatization in your tank.

But to be safe, do you have your water params?

 

Just checked my levels again and this is what they are at

P.h 7.8
Alk 300
Nitrite 0
Nitrates 30

My ricrodea very well could have been dying, the guy i bought it from had a hard time scrapping it off his live rock, but the xenia deffianetly lost its stem and some how got removed from the rock. I used super glue gel to attach them. and the kenya is super glued to the coral plug the others were glued directly to the rock