Dying zoanthids

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Aquarium Advice Apprentice
Oct 20, 2005
Baltimore MD
After doing fine and multiplying for 5 months, my zoanthids all of a sudden started closing up and dying. I maintain Ca at 450-480, ph 8.1-8.2 depending on time of day. I do regular water changes of 10% every other week. I have a large heavily lighted refugium, and a prism skimmer. The system total volume is 120 Gal.
All corals, fish, anemones and iverts are doing fine. I even have a colony of star polyps, which are quite close to zoanthids from what I understand, and these are doing great, its just zoas that are vanishing.
Any ideas what to look for?
The problem is that this is happening all over my tank. I had 6 clusters of zoas scattered all over, and i only have a handful of zoas left that are alive. Some of them were on a rock with a larger (predatory-type) green polyp on it, and he's doing fine.Some of them were alone, and they are also dead. It does seem to be a water issue, but I have done all tests I could find in the LFS, and nothing seems to be wrong. pH, Ca, dKH, temperature, salinity, iodine, all within norms. No detectable nitrates, nitrites, or ammonia. I am at a total loss.
Have you inspected the polyps at night? Maybe an hour or so after lights out. Maybe you have some predatory snails or crabs. Or, seaspiders. They will also suck out the life of soft corals.
Otherwise, you could try doubling up on your water changes and see if that helps, even if the water tests are fine, it can't hurt.
No anges. I only have two mandarin gobies and four damsels (two blue and two fiji), I am not big on fish. I like inverts and corals.
I did look at the during the night, nothing suspicious. The only remotely possible suspect is a pistol shrimp that was a hitchiker on LR. I do have blue legged hermit crabls, but have not noticed them doing anything to the zoas.
What is your PO4 reading?

Too much PO4 can cause tissue breakdown and cause the colonies to deminish...


There may also be a parasitic problem... Do they look slimey? Are they Phosphorescent in the actinics? If the Zooxanthelae die off, the coral will starve and thus deminish the colony...
My PO4 is 0.25-0.4, its hard to say more precisely with the nutrafin test I am using. This is admittedly non zero, but should not cause tissue breakdown.
I do have a slight cyano problem, and some of it is on the zoas. This does indicate some overfeeding on my part. I will try to do bigger water changes, and skip more days in a week feeding my corals. Anyone thinks this might be the cause of the zoas closing up and slowly vanishing?
Yes, cyano will cause the polyps to stop opening and slowly die if left unchecked. Keep the polyps cleaned off as much as possible. I had a bad outbreak and cyano killed off some of my xenya. It can wreak havoc on a reef tank.
I would try to see if you can spot some zoanthid-eating nudibranchs. They can be pretty hard to spot since they take on the color of the zoos that they have been feeding on. Also they have a skirt that helps them fit in with the colony. Try doing a google search on them and I'm certain you will be able to get some info on these guys.
my buddy had sea spiders eating his... check at night......
if the cyno is not growing directly on the zoos the cyno probably is not the problem. I had cyno bad and my zoos all lived through the cyno. Like Fluff says you need to blow/suck the cyno off the coral otherwise they will die....

What type of salt are you using? I use Oceanic.....
I lost 96% of my zoos and my friend just told me that he read that high magnesium levels will kill off zoos... I have not tested my water yet so I can't be 100% sure if thats a cause. Just something else to cross off the list.
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