Hammer Coral Not Opening – Reasons & Solutions

So, your hammer coral is all closed up and not opening, and now you are starting to question if you are taking care of it correctly… Well, try not to dwell on it too much, as I am here to solve your problems!

Hammer corals are one of the most beautiful LPS (large polyp stony) corals in the reef aquarium hobby. When it comes to corals for reef tanks, hammers tick all the boxes. 

Not only are they pretty hardy corals, but they also come in fantastic colors, and they will add lots of movement to your aquarium. Put it this way, you can’t call yourself a true reefer until you get your hands on a mesmerizing hammer coral!

Even though the hammer coral is considered ‘hardy’, they often cause issues for beginner hobbyists. One of the most common problems is hammer corals not opening, and while it may seem that your hammer coral is not opening for no apparent reason, usually there is an underlying condition that you missed. 

So, with that, let’s dive into the most common reasons your hammer coral may not be opening.

Why Is My Hammer Coral Not Opening?

There are many reasons why hammer corals decide to stay closed and not open. It could be an issue with their environment, such as insufficient water flow or poor lighting, or fluctuations in the water parameters. Or, it could be invasive algae or pests irritating your hammer coral. 

If your hammer coral is not opening, the issue could be:

  • Your hammer coral is taking time to adjust to its new home 
  • Fluctuations in the water parameters
  • Lighting issues
  • Water flow issues
  • Pests
  • Algae


Newly added hammer corals need time to adjust to their new environment, so don’t be surprised if your hammer appears irritated for a couple of days. Some hobbyists have found that it can even take up to three weeks for hammer corals to fully open. 

The most important thing is to not lose hope and give up, and to remember that most reef hobbyists have different setups. As long as you maintain stable water parameters and acclimate your hammer coral to your lighting and water flow, there should be nothing to worry about. 

Water Parameters

LPS corals will be the first to tell you when the water parameters are off inside your aquarium. Even though hammer corals are relatively easy to care for, they are sensitive to large swings in their water parameters. 

If your hammer coral is not opening, you should test the water immediately, to accurately measure the water quality and to check if any adjustments are needed. 

You should start with temperature and salinity because when the salinity and temperature levels are off, usually all the other water parameters will be too. Hammer corals require salinity levels between 72 and 78 °F, and alkalinity should be 8-12 dKH. 

If your salinity and alkalinity levels are on point, check the calcium and magnesium levels. Hammer corals require calcium levels between 350 and 450 ppm, and magnesium 1200-1350 for healthy growth. 

Nitrates and phosphates are also important when it comes to hammer corals. Nitrates should not exceed 10 ppm and phosphates should remain at 0.01 ppm. Elevated nitrates and phosphates can soon become a headache for both you and your hammer coral, but on the other hand, your hammer coral will not be happy if the nitrate and phosphate levels bottom out. It is important to find a balance and keep them stable. 

Reef Tank Advisor’s Top Tips For Controlling Your Water Parameters

  • Test the water parameters at least once a week.
  • Perform a 25% water change every month, or if your aquarium is heavily stocked with corals and fish, a 15-20% water change every 1-2 weeks is recommended.
  • Do not overfeed your aquarium, and siphon any uneaten food.
  • Remember to keep on top of changing and rinsing your filter media. 
  • Use a dosing pump to control the calcium and alkalinity levels. 


Sometimes, lighting can cause your hammer coral to not open. Lighting shock from poor acclimation can cause your hammer to remain closed, so always start by placing your coral on the sandbed, and slowly move it up onto the rocks if you want to add it to your aquascape. 

Your hammer coral requires moderate lighting, with a PAR range between 80 and 150 for optimum growth and coloration. 

Water Flow

Water flow is also a factor that can affect your hammer coral opening or closing its polyps. You should provide your hammer coral with moderate flow, give it too much flow, and it will not open. 

For example, if you blast your hammer coral with strong water flow, it will retract its polyps to prevent them from tearing. You want enough flow for your hammer to gently sway in the current, but not too much that it is bending over its skeleton and/or not opening. 


Sometimes it is nothing to do with your aquarium’s conditions, but instead, your hammer coral may not open due to an intruder or irritating critter. 

For example, peppermint shrimps are known for nipping and climbing over LPS corals, even though they are great additions to reef tanks to remove aiptasia anemone pests. Fish that are not considered reef-safe can also pose a threat to your hammer coral. 

So, observe your reef tank, especially during the night, and see if anything is bothering your hammer coral. If you are ever unsure about reef-safe inhabitants for your hammer coral, speak to your local fish store. 


Some green algae are considered ‘good’ algae, often indicating good water quality, but nobody wants to see the algae growth get out of control. Excessive algae growth not only deprives fish of nutrients, but it can also make your hammer coral close its polyps. 

Luckily, if your hammer coral is not opening due to algae, it is an easy fix!

Simply remove your hammer coral from your aquarium and gently clean the algae off. When doing so, be careful not to damage any of your hammer’s tissues. I recommend using a soft toothbrush (the ones designed for children work wonders) and a turkey baster to blow the algae away from your hammer coral’s soft tissue areas.

How Long Will It Take For My Hammer Coral To Open?

After you have identified the problem and resolved it, it can take a couple of days to a few weeks for your hammer coral to open back up again into its normal waving self!

If after a few weeks, your hammer is still not opening, go back to the previous section of this article and go through the different reasons why your hammer coral may retract. 

My Hammer Coral Is Showing Its Skeleton

If your hammer coral is not opening and exposing its skeleton, then it is slightly alarming. Let’s just say, when a coral shows its skeleton, it is never a good sign. If your hammer coral looks this way, it means it is not happy, and it is probably dying. 

In most cases, it is a lost cause, however, do not give up hope quite yet… In the early stages of an exposed skeleton, you can still save your hammer coral. 

Immediately remove your hammer coral from the aquarium and dip it in an iodine solution for 15 minutes. If your hammer coral still has healthy heads, frag them off and place them in an area that meets its lighting and flow requirements. 

Keep your water parameters stable, keep a close eye on your hammer coral, and hope that all your hard work pays off. 

Where To Place Your Hammer Coral Inside The Aquarium?

To prevent your hammer coral from becoming unhappy and not opening, getting the placement right from the get-go is really important. 

Hammer corals are not too finicky when it comes to placement inside aquariums, as long as it is receiving the right amount of lighting and flow, then they should be happy and healthy!

Following your hammer corals requirements, you should place your coral at the bottom of the reef tank in the sand bed, or on a rock in the middle region. 


It can be scary and frustrating when your hammer coral does not open for a long time, however, there is usually a good reason for this behavior. There is usually a solution for every problem we experience in coral husbandry, so do not panic. 

Check the water parameters are stable, and the lighting and flow requirements are on par with your hammer coral’s needs. If that all looks good, then check if anything such as pests or algae is irritating your coral. 

Once you identify the issue and resolve it, your hammer coral should open up again within a few days or weeks, so sit tight, and look forward to your hammer showing off 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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