Why Are Zoanthids Dying in My Reef Tank?

For newbies to reef-keeping, Zoanthids, or Zoas, are some of the easiest, hardiest and most interesting creatures to have. But, sometimes their polyps seem to stay closed and there are a plethora of things that can cause them to appear as if they’re dying.

So, if you’re wondering why your Zoa is dyinig, understand what you’re seeing doesn’t necessarily indicate death. But, if you think it’s coming to the end of life, it could be due to poor water parameters, bad placement in the tank, another fish is preying on it or it’s not getting enough nutrients.

Why won’t my Zoanthids open?

The biggest reason why Zoanthids stay closed is because a hermit crab, cleaner shrimp, or other invertebrate crawls over it. Understand, though, this doesn’t automatically mean your Zoa is dying.

Another reason could be due to a fish picking at it, thus causing the Zoa stress. Poor water parameters, such as fluctuations in salinity or pH balance, are another cause for Zoas closing.

If the Zoanthid is new to the tank, the polyps stay closed until it acclimates to the new surroundings. Give the Zoa time to adjust and the polyps should open soon.

What are the signs of Zoanthids dying?

Signs of Zoanthids dying will include things like not opening for longer than is normal, discoloration, or the appearance of melting. But, these are tough creatures, so don’t give up on them even if they show these signs.

Some of the indicators to observe are:

  • Darkened coloration or pigmentation
  • Disintegrating polyps that look like they could come apart
  • Appearance of wilting or collapsing
  • A foul odor emanates from the tank

Why are my Zoanthids dying in a reef tank?

It’s important to note that, although most people understand Zoanthids to be corals, they’re not. They’re cnidarians and, in scientific circles, are colonial anemones. So, in essence, they are soft corals, which isn’t the same as regular coral. They are both plant and animal, so they need a balance of food and light.

One Zoanthid is actually many individual creatures living in one body. Therefore, you’re taking care of multiple beings in one organism. If you notice your Zoa dying in a reef tank, it could be due to the fact that you’re treating it like a regular coral.

How do you save dying Zoas?

The first thing you should do is check the water parameters and take care of any imbalances right away. Remember, water fluctuations will cause Zoa stress. Ensure the water parameters include:

  • Temperature:  78°F
  • pH Balance:  8.0 to 8.4
  • Salinity:  1.025 (use a high-quality salt mix)
  • Hardness:  7 to 12 dKH
  • Light:  use a quality reef LED, metal halide, or T5; the Zoa should sit in mid to high ranges of brightness to allow for polyp growth, color development, and photosynthesis for the production of the Zooxanthellae.
  • Water Flow:  low to medium movement of water with a varied pattern. Too high will cause stress and health problems along with stunted growth and dull color.


Once you have the water squared away, check your feeding schedule and the method of food delivery. Even though Zoas have symbiotic Zooxanthellae and they’re capable of capturing prey, target feeding is still essential. There are many commercial foods on the market perfect for Zoanthids.

There are a couple of ways to target feed. But many people prefer putting the food right in front of the powerhead before the light timer turns off. Other people will use a non-needle syringe and inject the food into the water just above the Zoa’s location. You could also try this alternate method for target feeding:

  1. Mix the recommended amount with some tank water until you have a thick paste.
  2. Turn off all air and water pumps.
  3. Fill a turkey baster or other large syringe-like device and place a bit of food on each polyp as close to the center as possible.
  4. Turn the pumps back on and observe the Zoa.
  5. If it’s not eating, try changing the food. You can also try giving more or less food to see if that helps.
  6. You’ll know they’re full when you see a dark brown substance around them a day or two after. This isn’t a brown jelly disease and it doesn’t mean they’re dying; it’s simply defecation coming from its mouth.


If your Zoanthid is melting away, check its light exposure, water flow, and water parameters. Too much light, too high water pressure, or poor water parameters will cause melting.


If you’ve done everything you can and you still notice your Zoanthid dying, try putting it in a different spot within the tank. Make sure they get an equal balance of water flow and light exposure.

What does Zoa pox look like?

If you notice your Zoa covered in tiny growths with the polyps staying closed for a long period of time, they could be sick with Zoanthid Pox. If you use an antibacterial treatment, like Furan-2, it should clear up the situation quickly. But, you have to do this right away if you hope to save your Zoanthid.

If you have to, call a vet specializing in aquatics and do a bit of research through the various online forums. Many people have advice for this and some treatments will be specific to the species of Zoa you have.


If you believe your Zoa is dying, always ensure the water parameters are appropriate before taking any other measures. But, if you notice growths along with closed polyps, it could be suffering from Zoanthid Pox. In which case, you’ll have to administer some antibacterial medication as soon as possible.

However, if the polyps are only closed and everything else checks out, including their ability to eat, then it probably isn’t much of a concern. The only time you should worry is if the polyps have closed for longer than usual, if there’s noticeable discoloration or they appear to be melting.