bubble deflation

a couple of days ago one of my two bubble corals just deflated until all
that was left was a white skim on a piece of rock. there were no water
quality issues that I can pinpoint other than the removal of about 24 sea
slugs that may have been plundering my corals.
now today it's pal has done likewise but after a few hours it has returned a
little to almost half it's original size. I can see a few very small
bubbles on the other piece and I'm wondering what would cause them to go
downhill so rapidly, one day great and within a few hours totally deflated.
the only thing I can see is a spike in the nitrates to 10ppm but that would
hardly cause this would it?
my xenia are all closed up to so have I released something into the water as
I captured the slugs (with tweezers) but the mushrooms etc... are fine and
larger then ever with the button polyps popping up all over the place.

Robin