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Why Is My Coral Turning Brown?

by Brian Dunleavy
Why Is My Coral Turning Brown?

Corals typically turn brown when the overproduction of microscopic algae called zooxanthellae happens inside a coral’s tissues. The zooxanthellae block the coral’s natural pigments, giving it a brown hue. 

Are you worried about your coral because it has suddenly turned brown?

Corals turning brown is often a sign that something may be wrong inside your saltwater reef aquarium. This can quickly happen because corals are easily stressed by minor environmental changes.

Why Do Corals Turn Brown?

The reason corals turn brown is the overproduction of zooxanthellae. These photosynthetic algae live inside the coral’s tissue and give the corals their beautiful coloration. 

The cells of the zooxanthellae are naturally brown to golden-yellow. When the zooxanthellae population increases, the coral takes on a brown color due to a decrease in chlorophyll pigmentation, which is green. This overpopulation causes the natural pigments of the coral to become blocked, resulting in the coral turning brown. 


Zooxanthellae also play a critical role in the growth and productivity of corals. Corals have a mutual relationship that benefits both the coral and algae. The corals provide refuge for the zooxanthellae, and in return, the zooxanthellae supply oxygen and remove waste products for the coral.

Typical Reasons For Corals Turning Brown

As mentioned, corals turn brown from the overproduction of zooxanthellae, but how does this exactly happen inside saltwater aquariums?


Before your coral enters your aquarium, it has to go through transportation, which increases stress for your coral. This can be from your local fish store (LFS) or online shop to your home, or removing it from the aquarium to frag it. 

Any browning after transportation is usually normal. During this time the coral will be without lighting, causing the zooxanthellae to slightly overpopulate, turning the coral brown. 

What Can You Do?

  • Be patient - corals typically color up after acclimation
  • Get the placement right the first time to prevent moving your coral 
  • When fragging corals, perform the process quickly


Elevated Nutrient Levels & Water Parameter Swings

Elevated nutrient levels result in zooxanthellae multiplying like mad! So, best keep those under control at all times.

What Can You Do?

  • Test the water parameters 
  • Perform regular water changes with reverse osmosis (RO) water
  • Do not overfeed and check the coral food doesn’t contain preservatives like phosphate
  • Siphon uneaten food
  • Use a phosphate media to remove phosphate
  • Add carbon to your filter 

FUN FACT: Elevated phosphate levels inside reef aquariums and the wild can also contribute to coral bleaching.

Poor Aquarium Lighting 

Corals need correct lighting to survive - this is why most brightly colored corals are in the upper part of coral reefs in the ocean!

The light spectrum of your aquarium lighting plays a critical role in the biological processes of corals inside your reef tank. Poor lighting can inhibit the growth and photosynthesis process of zooxanthellae, causing them to overpopulate. 

What Can You Do?

  • Increase the light intensity*
  • Buy a PAR meter to monitor light levels 
  • Upgrade the light fixture 
  • Remember to replace bulbs every 6 to 12 months

*When increasing the light intensity, be careful not to blast your corals. Blasting corals, especially corals that require low-light environments, increases coral bleaching.


Can You Prevent Corals Turning Brown?

coral turning brown

If your coral is turning brown, you are likely wondering how you can prevent it from happening again. 

The best solution is to maintain tank stability. Stable water parameters and water chemistry are key when keeping corals inside saltwater aquariums, particularly slightly less forgiving corals like small poly stony (SPS) corals. 


How To Control Tank Stability?

  • Control nutrient levels and keep them constant
  • Frequently test the water parameters
  • Control the aquarium lighting
  • Keep a deep substrate
  • Introduce live plants
  • Never overstock your aquarium
  • Perform regular water changes, but do not over-clean
  • Keep the aquarium out of direct sunlight

How To Control Nutrient Levels And Water Parameters Inside Your Aquarium?

Elevated nutrient levels and fluctuation in water parameters are often the leading cause of coral turning brown. 

Depending on what coral you are keeping, will determine the ideal water parameters. However, for most corals, the ideal phosphate levels are between 0.01 - 0.05 ppm and 0.025 - 10 ppm for nitrates. If your nitrate and phosphate levels are too high, perform a 25% water change immediately and retest the water until they are under control and within range. 

If water changes are not doing you any favors, you can try the following:

  • Apply nutrient removers like a refugium
  • Use a phosphate media reactor - these are the most successful way to reduce nutrient levels 
  • Dose the aquarium water with a phosphate remover

While dosing and installing equipment will help keep nitrate and phosphates under control, often it only takes improving your maintenance regime to have a positive effect on improving your coral’s coloration.

In addition to nitrate and phosphate, you want to watch the calcium, alkalinity, and magnesium levels don’t creep up or dramatically fall out of the recommended ranges for reef aquariums.

  • Calcium: 380-450 ppm
  • Alkalinity: 8-12 dKH
  • Magnesium: 1180-1460 mg/L

Calcium and alkalinity are particularly important if you have SPS or large polyp stony (LPS) corals. SPS corals like Acropora corals require these parameters to grow their calcium carbonate skeleton, and to also prevent them from turning brown.


How To Control Lighting Inside Your Aquarium?

As mentioned, poor lighting is another attribute that causes corals to turn brown. 

Coral frags from your LFS are likely to have grown and be kept under high lighting. So, when adding a new coral, monitor the health of your coral, and if your coral is turning brown, move it slightly higher up the aquascape, or increase the aquarium lighting by 3-5 % every week until the desired lighting level is achieved. 

We highly recommend getting yourself a PAR meter to measure the aquarium lighting level to prevent coral bleaching from overexposure to light. 

Is It Possible To Reverse Corals Turning Brown?

Absolutely. With the right aquarium conditions and time, you can restore their beautiful colors by following these steps.

Tips For Brightening Corals

  • Keep water parameters stable
  • Regularly check water parameters with a reliable aquarium test kit
  • Maintain low nutrient levels
  • Provide your corals with enough lighting
  • Be patient - corals do not color up overnight



Corals turn brown from the overproduction of zooxanthellae. This is because the zooxanthellae block the natural pigments inside the coral’s tissues. The coral then takes on the coloration of the zooxanthellae’s cells, which are golden-yellow to brown. 

To prevent corals from turning brown, tank stability is key. Even if the zooxanthellae population is not the reason your coral is turning brown, elevated nutrient levels can be. 

Remember to always test the water parameters and keep on top of water changes. Also check the lighting level is what your specific coral needs.


by Brian Dunleavy