Zoas dying need little help

It' could be Zoa eating nudibranchs or sundial snail causing the issue also could be from somthing falling on them. I would remove them and give them a coral dip with Coral Rx and that would also be a good time to look them over good some pest have to be removed with tweezers. That would be where I would start also wear protective gloves Zoas can get nasty. Hope that helps a bit keep us updated

 

It could. Those anaerobic areas of the sand bed can contain some nasty compounds.

 

You seem to have found the problem but I have had the same issue from time to time. I had a nice colony of radiocactive that had probably 30 heads, then they started disappearing by the day. I took pictures and could compare, at one point there was 8. I watched at night and what looked like pods crawling all over them. Got a mandarin and problem went away. But I did have a rock of some orange zoas, about 20 heads, they just disappeared. Didnt turn color, didnt melt, just gone. Down to 2, then 1 then none. Could be the type of zoa, I have a frag of what appears to be ked reds, they all closed up and turned brown. They happen to fall in the sand so I left them, and 2 new ones opened, so I took a toothbrush and gently brushed the brown off, it came off like a scab and all new tissue underneath. I moved them and they did it again, all turned brown. I left them and the frag is at a angle, the top heads are coming back but the bottom arent. Ive been told some make it and some dont. I have noticed a black snail with a flat and small shell, but it hasnt gone near anything.