Most Common Issues for Coral in a Reef Tank

A frequent question that comes up among reef aquarium enthusiasts is, “What are the most common issues for coral in a reef tank?”

When an issue with a coral arises, we go through many steps to try and figure out what the cause might be. If you’re a novice to the hobby or simply stuck on how to handle a particular coral issue, perhaps this guide can help you! Below are some of the most common causes of coral issues.

#1: Coral is Losing Flesh / Tissue

Commonly called tissue necrosis, it is determined by how fast the tissue peels off the coral skeleton, Rapid Tissue Necrosis (RTN) or Slow Tissue Necrosis (STN).  In RTN, a coral generally loses its tissue within 24 hours.  STN can take days or even months.

The Cause: There are many issues that can cause tissue necrosis in corals. Here are some of the most common:

1. Sudden change in alkalinity
2. Consistently low alkalinity 
3. Drastic change in salinity
4. Significant temperature swing, up or down
5. Bacteria such as Philaster Lucinda and Philaster Guamense

The Solution: Slow Tissue Necrosis (STN) is difficult to stop and Rapid Tissue Necrosis (RTN) is nearly impossible to stop. Here’s what you can do to try and minimize the damage:

1. Make sure temperature, salinity, alkalinity and pH are all within the optimal range. Continue testing daily until resolution.
2. Cut off the impacted area(s) of the coral. To stop the spread of the tissue necrosis, cut into the healthy flesh of the coral above or below the impacted area.

#2: Coral is Turning Brown

The Cause: Corals typically turn brown as a result of the overproduction of zooxanthellae (a type of algae) inside of a corals tissue. As zooxanthellae levels increase, they block the coral’s natural pigments causing them to turn brown. This is typically caused by:

1. High levels of nutrients, like nitrates and phosphates. These nutrients act as a food source for zooxanthellae resulting in their overproduction.
2. Constant fluctuations in tank parameters.
3. For new corals, if the light levels in the new reef tank are significantly less intense than their previous environment.

The Solution: Overall tank stability is key.

1. Keep nitrates between 0.025 – 5.0 ppm and phosphates between 0.02 – 0.05. If these levels are higher nutrients then frequent water changes will help reduce high nitrate and phosphate levels. Adding a biopellet or GFO reactor will also help reduce these levels and keep them within range.
2. Regularly testing of key tank parameters and regular maintenance is essential. Learn more in our article, Water Quality for Reef Tanks (Part 1): The Most Important Parameters.
3. For new corals, carefully monitor their health. If they are turning brown, slowly move them up in the tank to increase the light intensity until they are happy.

#3: Coral Bleaching

When the tissue of a coral begins to lose color or becomes translucent or white it is known as bleaching. Unlike with tissue necrosis, the tissue itself remains attached to the coral.

The Cause:  Stress from a multitude of different issues can cause a coral to bleach including…

1. High temperatures which can cause the coral to expel its zooxanthellae turning the coral white. 
2. Lighting is too intense
3. Large swings in key tank parameters, like salinity, alkalinity and pH.
4. Levels for nitrates and/or phosphate are too low.
5. Levels for nitrates and/or phosphate are too high.

The Solution: Keeping all tank parameters within range and stable is important to prevent coral bleaching.

#4: Lack of Polyp Extension

When a coral is happy, their polyps will be extended and moving with the flow.  When a coral is stressed, they will have less or even no polyp extension.

The Cause: Many issues can cause a lack of polyp extension including…

1. Unstable water parameters
2. Lack of water flow
3. Too much water flow
4. Parasites and other bugs irritating the coral
5. Fish nipping at the coral

The Solution:

1. Routinely check all water parameters and make sure they are consistent and within range.
2. Make sure that the coral is in an area where they can receive the recommended amount of flow. In general soft corals and LPS corals need low to moderate flow and sps corals need higher flow.
3. Inspect and dip all corals before adding them to any reef tank. Make sure to remove, quarantine and treat any coral if any parasites are found. 
4. Make sure to add reef safe only tank inhabitants.  Many fish will nip at certain corals and are not recommended for reef tanks.

Avoid Common Coral Issues!

When a coral is stressed or is dying it may be from a single or even multiple issues.  Catching the first signs of stress and correcting the issue will help reverse and may even prevent coral death! In order to prevent such a disaster, always keep these most common issues for coral in mind!

Did we miss anything in this article? Have something to add? We love to hear from our readers, feel free to give us a shout here!