How long have you had these Zoas? Do you have more Zoas and a Palythoas? Are these the only ones doing this? How long have they looked like this?
It could be a water flow issue. It could be a light issue. It could be a water parameter issue. If not any of these, you could try adding Coral Frenzy or Reef Roids food and see if that works.
The thing I have learned about Zoanthids and Palythoas, and I have over 25 assorted kinds, is a particular frag or colony can look awesome for a period of time. Then they will shrivel, or close up for awhile. You might think that something was wrong and they were going to melt and die. All of the sudden, they reappeared as if nothing had happened.
Tank almost 95% zoas. All zoas about a year old. The rest of the zoas look fine.
What's pictured is on the very bottom of the tank.
What's strange is the damage to the zoas skirt tissue. Weird
Besides monthly WCs I DAILY dose for last 6mos in my 180g DT with a total water volume of around 280gals:
* 4ozs 80proof vodka
* Capful of Iodide which I think is 5ml
* 1/2 teaspoon Kalk
* 1/2 teaspoon of pharmaceutical grade potassium chloride
* 1/4 teaspoon of pharmaceutical grade strontium chloride
2-3 times a month I Test for:
* Calc - been around high 400s to nearly 500 (high bc of Kalk use)
* Alk - been around 10
* Mag - been around 1400
* Potassium - around 400
* NO3 - bouncing btwn 0 - 2
* I dont test for Strontium (I know, I know)
* I dont test for Iodide bc I've read many place the tests are very inaccurate to the true amt
Lighting:
Im pretty good with a soldering iron so I converted all my LED lights over to 50% 460nm and 50% 420nm.... Ditching a lot of the white LEDs. I did leave a rare few white lights but lighting is 98% 420-460nm LED lighting.
6ft tank covered end to end with about 1000watts of 420-460nm lighting. Too much ?
Light cycle slowly ramps up from 0% to 100%. Runs 100% for about 8hrs. Then slowly ramps down. 8-9hrs of complete darkness overnight.