Just want to share my disappearing Zoa saga. I have a large rock in the mid area of my 29 gal JBJ Intermediate LED tank and it measures about 13" across. Six months ago the whole rock was covered in Zoas. My set up has a hermit crab, a 6 yr old Clown, a Yellow Damsel and 2 types of Star Polyps. A few months ago I noticed my Zoas were disappearing. They would first close and then becoming covered in a brownish/green coating but sometimes they would just disappear overnight. Before long I was down to only two small areas (each roughly 2"x6"). Nothing new had been added to the tank over 3 years and I was sure I had not made any changes, but after reading other posts about Zoas dying I realized I had made a few changes that I'm afraid may have led to the death of my Zoas.
I had changed the brand of Salt I used.....went from Kent to Topfin.....and also added a packet of Chemi-Pure Elite in addition to Purigen to the overflow.
(For 4 years I had used only poly filter and some Purigen in the overflow.....and every now and then I'd add a mix of GFO & charcoal.)
The other change that occurred in the tank was the number of Pods multiplied to what appeared to be the hundreds. At night the Zoas would close and Pods would be covering every square inch of the tank.
Whether the Pods were eating the Zoas or just irritating the heck out of them, I don't know, but I switched back to Kent Salt, removed the Chemi-Pure and now have only the Poly and Purigen in the overflow. I still alternate Purigen with the GFO/charcoal mix.
Also, a small Six Line Wrass and a Cleaner Shrimp were added to the tank in order to get the Pods under control. Within days the Zoas stopped dying and a week later I saw new ones growing. Needless to say it's now difficult to find a Pod swimming around, but my Zoas look great!
I think the Pods were the main problem but it also possible the Chemi-Pure and the Purigen were just overkill for my small tank. Also, I'll never change from the Kent Salt again.....forgot how much I like it!