Why Are My Zoanthids Not Opening?

Are your Zoanthids not opening? If so, do not worry, this is a common problem faced by many aquarium hobbyists. The good news is, if you get everything back on track then you should expect your “aquarium jewels” to open back up within a few days!

Zoanthid Corals are hardy and generally easy to care for, making them great for beginners. However, sometimes something upsets them inside the aquarium, causing them to not open their beautiful polyps.

Possible Reasons For Zoanthids Not Opening

Zoanthid Corals are certainly not shy, so, if your Zoanthid is not opening, there is something not quite right. There are a few reasons why your Zoanthids may not open, but do not panic, they will not stay closed forever, plus there are many ways you can prevent it from happening again. But first, let’s explore the possible reasons for your Zoanthids not opening. 


The most common reason for Zoanthids not opening is usually their tank mates. Check that any invertebrates inside your aquarium are not crawling all over your Zoanthids. While your invertebrates get much enjoyment from the act, your Zoanthids, on the other hand, do not like it, which could be a reason they are not opening.

The most common tank mates are hermit crabs and cleaner shrimps. Invertebrates like these need to walk or run around your aquarium, so we can’t be too angry with them. However, as long as they are not damaging the polyps, I wouldn’t worry too much as it shouldn’t take too long for your Zoanthids to open again. 

Cleaner Shrimp

Fish, however, could be a big issue depending on which fish you have in your aquarium. Reef fishes that are known to nip on corals should be avoided. Fish such as Tangs, Parrotfish, and some Angelfish are usually not compatible, so always check they are super-friendly before adding them into your aquarium with your Zoanthids. 

If you have noticed a particular fish picking on your Zoanthids, you can move the fish into another tank if you have one, or donate it to a friend that has compatible corals. 

Yellow Tang near Zoanthids

And then there are those sneaky pests that have likely hitchhiked their way into your aquarium…

If you notice Sundial Snails, it is best to remove them fast, as they love to munch on your coral’s pretty fleshy polyps. If your Zoanthids are going downhill one by one, these critters are likely hidden somewhere inside your coral’s polyps, so take a good look around! 

Many people dislike spiders, and so does your Zoanthid. Your Zoanthids will not open if you have Zoa Spiders crawling around your aquarium. These are easy to spot – they have eight legs and look like a regular spider you would find in your home. As many coral dips will not eradicate them, it is recommended to use tweezers to remove them.

Zoa Spider from CoralRX

Asterina Starfish are also an issue because they like to eat corals (especially zoanthids). Luckily they are pretty easy to spot, making them simple to remove. 

Lastly, If you are a scuba diver, you are probably a big fan of Nudibranchs – teeny tiny sea slugs that come in every color of the rainbow. However, Zoanthid Eating Nudibranchs are not your Zoas’ favorite house-mate. As Nudibranchs have many awesome ‘superpowers’, they can mimic the color of your Zoanthids, making them more difficult to identify. Once you do find them though, it is also likely they have left some gifts inside your coral – a massive number of eggs! They are definitely a pest that you do not want in your aquarium so take great care to make sure they don’t enter it in the first place.

To prevent these types of pests from hitchhiking into your aquarium, always dip your Zoanthids and quarantine them before permanently placing them into their new home.

Nudibranch on coral

Poor Water Conditions

If you were swimming in a dirty swimming pool, you wouldn’t want to open your eyes… That is exactly how your Zoanthids feel when living in poor water conditions, so in response, they protect their polyps by not opening. 

Any fluctuations in the water conditions could be the cause of your Zoanthids closing, therefore, you should always keep the following water parameters within range. Once the water conditions are stable, you will see those magnificent polyps again!

Ideal Water Parameters

  • Temperature: 75° – 80 °F
  • pH: 8.1 – 8.4
  • Salinity: 1.025 – 1.026
  • Alkalinity: 8 – 12 dKH
  • Nitrates: <10 ppm
  • Phosphates: <0.10 ppm
  • Calcium: 420 – 440 ppm
  • Magnesium: 1300 – 1450

While many hobbyists believe that nitrates and phosphates should be 0, this could be why your Zoanthids are not opening. This doesn’t mean your aquarium water should be dirty, but it does mean nitrates and phosphates should never be absolute zero. 

New Environment 

Are your Zoanthids new to your aquarium? If so, they may just need time to adjust to their new home. Placing your Zoanthids too early into your aquarium can be stressful, or even fatal. That’s why you should never skip the acclimation process before introducing a new coral into the aquarium. 

Some Zoanthids open within a few hours, while others may take a few days or weeks to fully settle in, and while it can be tempting to move them around and give them “some encouragement”, this is not the best way to approach the situation.

Incorrect Water Flow 

Most Zoanthids require a medium to high water flow, however, always check with what species of Zoa you have, as some have species-specific requirements. While higher flow helps Zoanthids open and keep debris off their polyps, flow rates that are too high (and too low) can make your Zoanthids unhappy, resulting in them not opening.

Incorrect Lighting 

It may be that your Zoanthids came from a lower light environment, so, as a stress response to higher lighting, they have decided not to open. Try moving your Zoanthids to a shaded area and gradually move them to an area with moderate lighting conditions, over a few weeks or months.


“Alle-what?!”. Allelopathy, put simply, means chemical warfare. When aggressive corals are placed too close to others they can be territorial, often sending off biochemical toxins that can influence their neighbors’ growth, survival, and development. 

Take a look at your Zoanthids neighbors and do some research to check their compatibility. 

Incorrect Location/Placement

Following acclimation, it is time to find the perfect placement for your Zoanthids! 

If your Zoanthids are not opening, and you have ruled out all the other issues mentioned above, then it may be your coral simply saying “please move me.” If this is the case, move them to a better-suited area, monitor their behavior, and hopefully, they will open up!

Zoanthids Still Not Opening – Is Your Zoa Dying?

If you have tried everything and your Zoanthids are still not opening, they are losing their color and seem to be melting away, it could be that they have started dying. 

Signs to look out for:

  • Wilting polyps. 
  • Shred polyps. 
  • Polyps losing shape. 
  • Foul odor. 
  • Darkened polyps or pigmentations on the tissues. 

However, do not give up quite yet, Zoanthids are hardy corals, so they may recover, just don’t forget about them!


Zoanthids are spectacular corals that are hardy and easy to care for, making them great for both beginner hobbyists and coral experts. Despite your experience, your Zoanthids may not open, which is an indication that something is not quite right. 

There are many causes, but the most common are poor water conditions, pests, poor lighting/water flow, and skipping the acclimation process. Hopefully, now you have the knowledge to identify the issue and prevent your beautiful Zoanthids from closing up again, creating that “Zoa garden” you always dreamt of.

  • Roy Lee

    I have an unhealthy obsession with reef keeping and maintaining successful tanks. If you haven't noticed from the website, I love everything related to saltwater tanks like coral, fish, and everything else in between.

    http://reeftankadvisor.com reeftankroy@gmail.com Lee Roy

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