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Why Is My Hammer Coral Not Opening!

With the hammer coral going viral on social media amongst the saltwater aquarium community groups recently, there has been a surge in the number of people reaching out and asking various questions about keeping hammer coral in their tanks.

One of the more common questions that we have seen people asking is why their hammer coral is not opening.

Although hammer coral is usually classed as a moderately challenging species of coral to keep, it is still usually very easy to keep if you have experience with keeping some of the more common beginner-friendly corals.

Still, there are a number of very common mistakes that we see people making time and time again when it comes to taking care of hammer coral.

If you do make one or more of these mistakes then there is a good chance of your hammer coral not opening at all.

Thankfully, most of these commonly made mistakes that we see people making time and time again are relatively easy to fix and the majority of people keeping hammer coral in their tanks should be able to remedy the problem quickly.

Why Is My Hammer Coral Not Opening!

The most common reasons that your hammer coral will not open include poor water parameters, fish picking at the hammer coral, a chance to your aquarium lighting, and an algae build up in your tank.

Although these do usually need a unique treatment, they are usually very easy to identify and treat in most tanks.

We will go through each of these potential problems that are preventing your hammer coral from opening below and go into how to identify and how to treat each one.

For the most part, this should be able to correct the problem that you are having and get your hammer coral to open again within a week.

Some issues are more serious than others and if the hammer coral is damaged or having problems with parasites or bacteria then it may need specialist treatments to help it recover.

Thankfully though, these tend to be rare and the problems listed above are usually the more common causes of hammer coral not opening.

Poor Water Parameters!

The most common reason that hammer coral will fail to open is due to poor water parameters. The key metrics that you have to monitor in your tank are the salinity, nitrate, ammonia, and po4 but the other metrics will come into play too.

You are able to use a salt water test kit to get a great deal of valuable information on the water parameters on your tank and highlight the potential areas where you may be having problems.

This is probably the best option to take due to here being a number of different things that can go wrong with the water in your tank that can prevent a hammer coral from opening.

Once you have managed to identify the water parameter that is out of range of what your hammer coral needs you can then take steps to fix the issue.

In some situations, your water test kit may confirm that there is nothing wrong with your water parameters and in that case you simply move on to the next section of our article.

Fish Picking At It!

Some fish will pick at your hammer coral and cause it to close and refuse to open unless the fish that pick at it are removed. This is rare but when it does occur, either the coral or the fish usually have to be removed from the tank as the fish will rarely stop picking at the corals.

It is generally easy to see when something like this is happening as you will clearly see your fish picking at your hammer coral. Even if the fish looks like it is innocently bumping the coral, this can cause it to refuse to open, especially if there are other minor issues in the tank for it.

Some fish such as angelfish do tend to be more prone to this type of behaviour but we have seen reports from a wide range of species picking at various types of coral. From what we can tell, there is no common link to predict if this will be a problem in your tank or not.

New Aquarium Lighting!

There are a number of reasons that you may have to change your aquarium lighting setup but this can be a reason that your hammer coral may refuse to open.

Too much light can be a common problem for the more sensitive coral species and it is very common that aquarium keepers will tweak their light setup and accidentally provide too much light to their coral.

Although this may sound like an easy problem to fix, it can involve a large amount of fault-finding, especially if you don’t have your old lighting setup anymore.

Doing a number of tests with different levels of light can be one way to correct this issue and get your hammer coral to open up again though.

If you have an entry-level light system for your aquarium then the specific output of the light may be difficult to work out.

If you have a more expensive lighting system then the manual will usually have all of the essential light output information that you require to match the light output with your hammer coral helping to save you a bunch of time.

Too Much Algae!

Both diatom and dino can sometimes build up on you hammer coral and cause it to close with the coral usually refusing to open again until the levels of diatom and dino are reduced. This does tend to be easy to fix but it can be a common cause of hammer coral not opening.

Keep in mind that if you do use an algae eater to deal with any algae or biofilm issues on your hammer coral, the coral may interperate this as a fish picking at it. This can cause the coral to stay closed for as long as a week after the algae have been removed.

Depending on your exact situation and the other corals that you have in your tank, this may be the best option to take but most of the time, a chemical solution or a coral dip will be used.

Thankfully, most people will be doing regular maintenance on their aquariums so algae build-up should be kept to a minimum so this is rarely a serious problem for most people.


That brings our article going over why your hammer coral may not be opening to an end. We hope that we have been able to help you better identify the potential causes of the issue with your hammer coral in your aquarium as well as help you understand how you are able to fix it. Unlike some other popular types of coral, hammer coral does tend to be easy to treat if it does refuse to open so you can usually get the issue corrected quickly.