Why Are My Zoas Turning Brown?

Corals turning brown is something even coral-keeping experts struggle with. As aquarium hobbyists we tend to be obsessed with coloration, and who can blame us! No one wants their beautiful Zoas to turn an ugly brown color!

If you are the type to search on reef forums, you have probably noticed how many times someone has asked why their coral has started turning brown. It is a fair question to ask. There is nothing more discouraging than seeing your beautiful Zoanthid losing their coloration.

Why Do Zoanthid Corals Turn Brown?

Before we get into why your Zoanthid has turned brown, first you need to understand how colors originate in corals. 

I am sure you would agree that corals are one of the most mesmerizing organisms on our planet. It is their coloration and intriguing nature which makes them so popular for hobbyists and why many people scuba dive. 

Zoanthid corals rely on lighting for coloration, however, many other factors influence coloration too. Zoanthids develop their colors to protect their tissues from strong UV rays and is also how they get their showstopping colors. Corals that live in shallower parts of the reef are more colorful than ones further down the reef in deeper waters. 

Your home aquarium is different compared to the ocean, however with new technologies, we can mimic these natural conditions the best we can. The color of your Zoanthid will depend on the light intensity and the zooxanthellae that live inside your coral’s tissues. 

Zooxanthellae are tiny microscopic organisms that have a photosynthetic relationship with corals, like your Zoa. Your Zoa provides protection for the zooxanthellae, and the zooxanthellae provide nutrients for your coral. Without this relationship, your Zoa would not be able to survive very long. 

Even though the zooxanthellae provide your zoa with nutrients, they are usually the main reason why Zoanthids turn brown. 

Zooxanthellae contain chlorophyll, a pigment that gives corals their colors. Naturally, the cells inside the zooxanthellae are brown, so without them, your Zoanthid would have VERY bright colors – much like when someone increases the saturation too much in a photo! 

So, if your Zoa starts turning brown, this indicates that the zooxanthellae cell population inside your Zoa’s tissues has increased. High levels of nutrients like phosphates and nitrates are usually to blame, as they provide nutrients acting as a food source for the zooxanthellae. In some cases, the coral may expel small amounts of the zooxanthellae, coating them in a brown-looking slime. 

Light intensity plays a large role in coral coloration as well. If the lighting and PAR are lower than your Zoa requires, the zooxanthellae will not be able to provide your coral with the nutrients they require for growth and coloration. As a result, the zooxanthellae cells increase, therefore, so does the pigment which has a brown color. So, in low light conditions, your Zoa will start turning brown. 

If low lighting is the issue, move the Zoas higher up in the aquarium or purchase a better lighting system such as full-spectrum LEDs. 

How To Prevent My Zoas Turning Brown?

The solution to Zoas turning brown is tank stability. Below are some tips to prevent you from coming home to a sad, brown-looking coral!

  • Keep nitrates and phosphates under control. Try to keep your nitrates between 0.025 and 5.00 ppm and phosphates between 0.02 and 0.05 ppm. If your levels are elevated, perform regular water changes and add a GFO reactor to help keep the water conditions stable.
  • Always test water parameters and carry out regular maintenance where needed. 
  • Always monitor the health of corals. If your Zoa starts turning brown, slowly move it further up in the aquarium to increase the light intensity. 

Tips To Brighten Up Your Zoanthid Coral

To keep your Zoa happy, healthy, and colorful, maintaining low nutrient levels is the first step. You can do this by reducing the amount of food you feed your Zoa and combining frequent water changes and an appropriate filtration system. 

As with any corals, your Zoa will brighten up if the alkalinity is within range. It is also recommended you monitor salinity, calcium, and magnesium because all of these water parameters can contribute to the zoanthid’s coloration.

Other trace elements such as iodine, halogen, and potassium have a direct effect on coloration as well. These elements can deplete rapidly, so always remember to stay on top of those water changes!


Many factors can influence your Zoanthids coloration, but the browning is all down to the overproduction of the photosynthetic algae that live inside your coral’s tissues, called zooxanthellae. 

So, don’t worry if your Zoa starts turning brown, coloration is not always down to poor coral health, the most common reason is incorrect lighting and water stability. The solution to healthy colors is to keep your Zoa in an area it will receive enough lighting, but not too much that your Zoa starts bleaching – that’s an issue for another day!

  • 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.

    http://reeftankadvisor.com reeftankroy@gmail.com Lee Roy

1 thought on “Why Are My Zoas Turning Brown?”

  1. I just took out my brown zoa and put it in a seperate bowl, brushed him reel good with paintbrush in bowl of saltwater, dumped water, put in fresh saltwater in bowl with zoa, brushed again, kept doing same procedure till zoa was green again. took 10 min


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