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Why Your Duncan Coral Has Closed Up And How To Fix It!

With duncan coral being such a popular option for many reef keepers due to it being a peaceful coral that is very easy to care for, a huge number of people new to keeping reef tanks or saltwater tanks choose it as one of their first coral additions.

This results in a number of people reaching out each month and asking questions about caring for their duncan coral as well as a number of questions about how to cover come common problems with them.

Recently, we have noticed a spike in the number of people who we see asking about why their duncan coral has closed up and what they are able to do to get it to open again.

Due to the main four reasons that a duncan coral will close up usually being easy to identify and work on, we decided that we wanted to publish our own article going over what you should be doing to try and get your duncan coral to reopen for you.

We have arranged our article in descending order of the more common causes at the top to the least common causes at the bottom.

This should make it easier for our readers to find the cause of their duncan coral being closed but you can navigate the article via our table of contents below to quickly skip to specific sections.

Check Your Water Parameters!

The most common cause of a duncan coral being closed up is due to poor water parameters in the tank.

Although duncan corals are generally considered to be more hardy than some other coral types, if the nitrate levels get too high for it, the coral will close up and stay closed for a prolonged period of time.

To better identify what the potential issue is with your water parameters, we would recommend that you add a water test kit to your fish keeping equipment.

It will allow you to test the water in your aquarium to check its parameters and then gauge what could be going wrong and allow you to work on ways to fix it.

Within a few days of the water parameters being brought back in line with what your duncan coral requires, you are usually able to see the coral opening up again.

This will depend on how poor the water conditions were and how long they were in that state for though as you may need to wait longer if the coral has been sat in unsuitable water for an extended period of time.

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Is There A Clownfish In Your Tank?

Clownfish are one of the most common marine fish kept by fish keepers and they absolutely love to host duncan corals and will happily lay on the coral for most of the day.

The problem is that the friction of the clownfish touching the duncan coral is able to cause the coral to close up and stay closed until the clownfish leaves it alone.

This is a very common problem for tanks that have clownfish and duncan coral in them as a clownfish will usually opt for a duncan coral to host over the various alternative you add to the tank.

This is why we did not add duncan coral to our list of the best anemone and coral for clownfish to host due to the clownfish often causing the duncan coral to close up and hide its beauty.

Unfortunately, this really can be a pain to fix as a clownfish will usually try to stick to whatever it chooses to host in your tank until they are separated.

This will often result in you either having to keep your clownfish happy and leave the duncan coral in its closed state or to separate the coral and clownfish into separate tanks.

If you do choose to separate the coral and clownfish then you can try a fake anemone to replace the duncan coral in your clownfish tank that works better than most people realise as covered in our article on using fake anemone with clownfish.

Are Any Of Your Fish Nipping At Your Duncan Coral?

Some fish species will happily nip at the corals in your reef tank with angelfish often being the common culprit time and time again but other fish will also often nip at some corals and force them to close.

This may be the cause of your duncan coral closing but it can be difficult to identify as some fish will only nip at the coral periodically throughout the day.

Checking that all of the coral that you have in your reef tank with your duncan coral are considered reef safe and usually don’t nip corals is usually a good place to start.

More often than not, this will identify the potential culprit that is causing your duncan coral to close up but similar to the clownfish issue, the only usually treatment is to separate the two.

Depending on your setup, you may be able to quarantine any potential culprits that you think may be nipping at your duncan coral to see if the coral re-opens once the nipping fish has been moved to another tank.

If the duncan coral stays closed with the suspect fish out of your main tank then it is probably closing up due to something else.

Do You Have Shrimp In Your Tank?

Shrimp are an often overlooked cause of a duncan coral closing up and some larger shrimp may only have to walk over a more sensitive duncan coral to get it to close up.

You will usually notice that the coral will open by itself after a couple of hours if this is the cause though as it is not under any direct threat.

Due to the duncan coral often only closing up for a couple of hours if it is due to shrimp the majority of people will simply leave their reef tank as it is and accept that every now and then, their duncan coral is going to close.

We have seen some people remove the shrimp from their tank and this is a valid option if you want but the benefit that the shrimp offer your tank usually outweighs removing them to keep your duncan coral open all of the time.

A compromise between the two can be to put any larger shrimp in your reef tank into a different tank if possible as the smaller shrimp will usually not cause your duncan coral to close.

This may be a way that you are able to keep the coral open but still get the benefits of having shrimp in your tank by keeping the smaller ones.


That brings our article going over why your duncan coral closed up as well as how you are able to get it to open again to an end. There are some less common reasons that a duncan coral may close up on you but they really do tend to be rare with the majority of people usually having problems with one of the four issues above.