Why Is My Hammer Coral Not Fully Extending?

So, your hammer coral has been doing great, but all of a sudden its polyps are not fully extending, and you are starting to wonder if this is normal, or if you should be worried… 

Well, do not worry, as you are not alone. In fact, many hobbyists have run into this issue, and with a just few tweaks here and there, your hammer coral will be back to its very full and healthy self!

The hammer coral (Euphyllia ancora) is one of the most beautiful large polyp stony (LPS) corals for reef tanks. They are highly sought after, but they are considered a challenging coral because of their needs, which is where you may have slightly slipped up. But that’s ok, as this article will go into detail on every known cause for hammer corals not fully extending.

Why Do Hammer Corals Close Up?

A hammer coral will be the first LPS coral in your reef tank to tell you something is not quite right. From poor water conditions to their natural splitting process, there are many reasons your hammer coral may close up and not fully extend.

Water Parameters

Your aquarium’s water chemistry is one of the most important engines. Water chemistry governs every factor inside your aquarium, directly affecting the health and growth of your hammer coral, so maintaining stable water parameters is key.

Now, it takes a moderate level of skill to properly care for hammer corals, because Euphyllia corals require very stable tank conditions, and they will not tolerate large fluctuations in the water parameters.

As your hammer coral is part of the LPS coral family, calcium and alkalinity are very important for growing its calcium carbonate skeleton, therefore if these are unstable, it will affect your hammer coral’s growth rate, or even cause its polyps to remain retracted. 

The calcium levels should be 350-450 ppm, so, 400 ppm is just right. If the calcium levels drop below 350 ppm, your hammer coral will start to die off. 

Upon testing the water parameters, you should aim for the results to be in the following ranges:

  • Temperature: 72° – 78 °F
  • pH: 8.1 – 8.3
  • Salinity: 1.024 – 1.026 (1.025 preferred)
  • Alkalinity: 8 – 12 dKH
  • Nitrates: <10 ppm
  • Phosphates: <0.10 ppm
  • Calcium: 350 – 450 ppm
  • Magnesium: 1200 – 1350 

If you have tested your water parameters and everything looks great, it may also be worth sending off a water sample for an ICP test to check the trace elements. 

Poor Placement

Your hammer coral may not fully extend, simply because it is not happy where you have placed it inside the aquarium, and because it can’t move like mushroom corals when they feel uncomfortable, one way that your hammer coral communicates its issues, is by closing up. 

Saying that, hammer corals are not terribly fussy when it comes to placement inside your aquarium, as long as you avoid extremes and meet their requirements, your coral should thrive.

You want to avoid extremely bright/intense lighting areas which also have a very high current, as these extreme conditions will cause your hammer coral to not fully extend and possibly, start to bleach.  

On the flip side, if you place your hammer coral in an area that is too dark and/or the flow is too low, you will also run into issues. 

For these reasons, most hobbyists will place their hammer coral in the middle of the aquarium on an exposed rock, or the bottom region on the sand bed.

Poor Lighting 

Following suit of most reef-building corals, hammer corals have a mutualistic relationship with symbiotic algae called zooxanthellae. The zooxanthellae play an important role in your hammer coral’s health, because the algae perform photosynthesis, providing your coral with 75% of its nutritional needs. 

For photosynthesis to happen, hammer corals require moderate lighting with a PAR range of 80-150. If the lighting is too intense, then your hammer coral is likely to retract its polyps, and therefore, you will notice it not fully extending. LED lighting is best for growth, coloration, and the overall health of your hammer coral, but give it too much white light and it will become irritated and shy away from the lights. A maximum of 40% white light is recommended for hammer corals. 

Insufficient Water Flow

In addition to lighting, water flow is critical for your hammer coral to be happy and healthy. A moderate water flow is recommended to gently sway your hammer coral’s polyps, but not too powerful that it puts pressure on the polyps. 

When the water flow is too powerful and/or direct, your hammer coral may start to bend over its skeleton. In the worst case, the high water flow will tear your hammer’s polyps, and in the best case, it will cause them to not fully extend. 

Lacking Nutrients

In addition to light, corals require additional nutrients. As your hammer coral has a large mouth, you should be feeding it meaty marine food that also contains amino acids. Amino acids are used for coral growth, tissue repair, and enhancing coloration. 

Despite their large mouth, hammer corals are fairly subdued eaters, but wave a piece of mysis or brine shrimp near its opening, and you should soon see them fully extending their polyps again.

Dead hammer coral

Irritation From Pests Or Tank Mates

Often when hammer corals are not fully extending, it is nothing you are doing wrong, instead, something inside the aquarium may be bothering your hammer coral. This can be a mechanical irritant such as fish nipping or invertebrates crawling over your hammer coral’s polyps, or something more worrying like a biological pest. 

Tank inhabitants, such as peppermint shrimp, angelfish, butterflyfish, and rabbitfish, are all culprits for polyps not extending. If your hammer coral becomes irritated, remove the offender immediately to avoid any further issues. 

Elevated stress levels from pests, will cause your hammer coral to not fully extend. 

Common hammer coral pests include:

  • Acoel worms
  • Euphyllia eating flatworms
  • Aiptasia sea anemones 

The same goes for these guys – if your hammer coral is being bothered by any of these, you need to remove them. If you are unsure how, have a read on pest treatments in our ultimate hammer coral care guide.

Aggressive Neighbors

Hammer corals are known for their aggressive nature, taking swipes at any coral that gets too close. However, they are not the only coral to misbehave, so, your hammer coral may not be fully extending because they are taking cover from their aggressive neighbor. 

Also, if your hammer starts chemical warfare, then corals that can, will fight back. Additionally, if you are not running carbon, the chemicals released from the warfare can increase your hammer coral’s stress levels resulting in polyps that are not fully extending. 

Hammer Coral Splitting

Sometimes, your hammer coral may not extend because it is in the middle of splitting. Splitting is a natural reproductive process where the hammer coral splits down the middle of its mouth, splitting off in separate directions, to create two separate corals. 

If your hammer coral starts splitting, it may tuck in its polyps for a few days. But do not fear, as once it has finished, your hammer coral will go back to its normal self!

Hammer coral splitting

Poor Judgment When Buying

Lastly, one that we often do not think about, is picking out the wrong hammer coral from the beginning. 

Sometimes our poor judgment when buying corals from our local fish stores or online can become a disaster. This is more common among beginner hobbyists that do not know what to look for when buying hammer corals.

When buying a hammer coral, you want to look for big and fat polyps that are fully extended, always avoid coral colonies that have retracted polyps. You should also pay attention to the color of the hammer coral. A healthy hammer coral will have vibrant colors, so shy away from any discolored, pale, or white coral frags. 


A hammer coral that is not fully extending is usually bothered by something, whether it is a mechanical irritant such as fish nipping, something biological like a pest, or chemical (for example, unstable water parameters). However, your hammer coral may also naturally retract its polyps when it is in the middle of splitting. 

The best approach is to go through the list, one by one, to identify the issue. Once identified, you can remove the issue (if needed) so that your hammer coral will extend its polyps again.

  • 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.

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