Amphipods are eating a zoa colony ...

Any omnivore will take pods as a food source. My damsels and maroon have completely wiped out the amphipod colonies in my 20G tank. I am lucky to see even one crawling around the rocks at night.
Remember that looks can be deceiving. Your zoas may have looked healthy but could be on it's last leg. Anemones are the same too, they look great one day, then completely melted the next. If you need to get rid of some of the pods, get a turkey baster and suck them up at night when the lights are out.

 

Yeah thats what I figured, the Zoas must have had some kind of issue .. The wierd thing is that even when each polyp was chewed almost completly through at the base, it was still open and looked OK ... I hear what your saying about them looking ok untill its too late , i just wouldnt think if they are dieing they would still be open and happy looking ... Thanks for the Baster tip, I will try that tonight ...