Acan Corals Not Opening? 9 Reasons Why

Acan corals are known to be easy coral to care for and are a great choice for beginners.

They can handle less than ideal water conditions and come in a variety of colors and sizes.

Acans are also one of the most commonly kept LPS corals and although they are a good option for beginners, many advanced hobbyists keep them as well.

There is a wide selection of Acans available including the Rainbow, Aussie, and Lord. 

Although Acans are a hardy coral that doesn’t require pristine water conditions, they are still prone to not opening up from time to time.

In this article, I will show you the reasons why your Acan corals are not opening.  

How do you know if Acan corals are not open? 

You will know that your Acan coral is not open if its skeleton is showing. Acans are normally round and puffy and won’t show its skeleton if it is healthy.

This is a common sign to look for when Acans are not open. If the skeleton is showing, there is something wrong.

Always confirm that no skeleton is showing on the coral before buying it.

9 Reasons why Acan corals are not opening:

  1. Lighting
  2. Water flow
  3. Acclimation
  4. Aggression
  5. Placement
  6. Fish nipping at them
  7. Fragging
  8. Changes in water quality
  9. Move to a new location

Acan corals not opening

#1. Lighting

One of the main reasons why Acan corals are not opening is due to lighting. A common mistake that is made when keeping Acans is giving them too much light. 

Acan corals prefer low to moderate lighting. High lighting will cause coral bleaching and the coral will stay closed up or even die. The proper amount of lighting is crucial for the growth and the overall health of Acan corals. 

If you notice they are not opening, make sure they are positioned properly to only receive low to moderate lighting. Keep them away from intense lighting and you will notice Acan corals will open up more. 

#2. Water Flow

Another reason why Acan corals won’t open up is due to improper water flow. Acans prefer to have low to moderate flow that is not pointed directly at them. A varied flow pattern that moves water around the coral is better than direct flow. 

If they are placed in an area of high flow, they will stay closed and not open up. Too much water flow will cause them to be stressed and make it difficult for them to pull food out of the water. 

big purple acan coral

#3. Acclimation period

When acclimating corals into a new tank, it is common for them to be stressed for a period of time. It can take a while for them to get comfortable in their new environment and they can show signs of stress. 

Once they settle in and get used to their new water and surroundings, they will begin to open up more. This can take anywhere from a few minutes to a few days. 

Read: Green Star Polyps not opening (11 Reasons Why)

#4. Aggression

Acan corals are generally peaceful but they can sting others if threatened. It’s important to leave enough room between them and other corals to avoid stinging or being stung

If Acans are stressed due to aggression, it is common for them to stay closed up until the aggression stops. This can lead to serious health problems and even death if not addressed quickly. 

#5. Placement in the tank

Proper coral placement is important for all corals, Acans included. They will be happy if you place them in the right spot in your tank.

Give them plenty of space between aggressive corals that can sting. Also, ensure that they are placed in a suitable area to receive the right amount of flow and lighting.

Acans can be placed on top of rocks or in between them. They can also be placed on the sandbed. If not placed in the proper place, it is common for them to not open up as they will be stressed.

Red Purple Acan coral

#6. Fish nipping at them

One of the concerns with keeping Acan corals is that certain fish can nip at their fleshy polyps. You should keep polyp nippers away from Acans.

If you are keeping reef safe fish that don’t bother corals, then you will be fine to keep Acans with them. But if you have coral nippers, this can lead to the coral not opening due to stress.

This can be a serious problem that will lead to the death of the coral if not addressed quickly. 

#7. Fragging

Fragging Acan corals can be done, but they are not considered to be an easy to frag coral. You will need to have the proper fragging tools and cutting in between multiple heads through its skeleton can prove to be a challenge.

Of course, if you are fragging these corals, expect them to be stressed and not opened. They will stay closed up for some time after fragging and will begin to open again while healing. 

#8. Changes in water quality

As mentioned, the Acan is a hardy coral and doesn’t require perfect water conditions to be healthy. But most corals are sensitive to water quality issues and stability is very important.

Anytime your water quality changes, expect your Acan corals to not open up for a period of time. Any change in water parameters can cause coral stress.

Temperature changes, nitrates, phosphate, ammonia, calcium, and alkalinity levels are just a few examples. 

It’s important to maintain a proper maintenance routine and test water levels regularly to ensure your water parameters stay within the normal limits.

#9. Moved to a new location in the tank

Whenever you are doing a water change or performing maintenance on your tank, it’s common for corals to be moved around.

Sometimes it is not possible for a coral to stay in its original position long term and eventually it will need to be moved.

Moving Acan corals around your tank can cause them to be stressed and not extend their polyps.

They will stay closed up for a period of time until they are comfortable again in their new location. 

How long does it take for Acan corals to open?

This depends on a variety of factors like lighting, water flow, water parameters, and others. But expect them to begin to open up anywhere from 15 minutes to 1 hour.  

Acan coral growth rate

The Acan coral is considered to be a fast-growing coral and is one of the main reasons why they are so popular.

It’s common for them to grow new heads on a regular basis, and they can grow from just a few heads to a full colony in about 1 year.


Acan corals are a great coral to keep in many reef tanks. They are colorful, grow fast, and are peaceful. If kept in the proper environment, expect them to stay healthy and open most of the time.

But any of the 9 items mentioned here can cause them to not open up properly. Following this guide will help you diagnose the reasons why your Acan coral is not opening up properly.