A typical, healthy photosynthetic coral has millions of tiny single-celled organisms, called zooxanthellae, living within its tissues. The coral and zooxanthellae live in a symbiotic relationship where the coral provides the Zooxanthellae algae with protection. In return, the algae provides food and oxygen for the coral and helps it remove waste. Coral tissue is transparent, allowing light to pass through it, and it is the millions of Zooxanthellae that live within the tissue that produce the pigments which give a coral its beautiful color.
What exactly is coral bleaching?
Bleaching occurs when a coral expels its zooxanthellae into the water column due to a single environmental stress or a combination of multiple stressors. Let’s take a closer look at some of the most common causes of coral bleaching in a reef tank.
Causes of coral bleaching in a reef tank
A bleaching event may be due to significant changes in temperature, light intensity, salinity, or prolonged problems, like insufficient water movement, malnutrition, or even stress-inducing chemicals, bacteria, viruses, or physical damage. Excess nutrients in the water can also exaggerate the problem by fueling growth in the zooxanthellae population to unhealthy levels making expulsion more common in the presence of an environmental stressor, like excess temperature.
Why bleaching is bad for a coral
It’s likely no surprise that bleaching is not good for your coral. The loss of zooxanthellae creates a massive hole in providing a coral with the proper nutrition. If a healthy zooxanthellae population is not regained/regrown, the coral will eventually starve. Coral bleaching is also associated with less production of protective mucus and increased chances of infection and tissue loss which can accelerate the colony’s decline.
How to prevent coral bleaching
Four essential things you can do to prevent bleaching are:
- Maintain stable water parameters: spikes in temperature or dramatic changes in pH, salinity, or other parameters may trigger an event.
- Acclimate new corals to your tank slowly, particularly if you have intense lighting, which most of us do.
- Keep nutrients in the form of nitrate and phosphate under control. Having some nutrients in the water is generally thought to be healthy for corals. However, excess nutrients encourage excessive growth of zooxanthellae, which could make your corals more likely to bleach in the presence of another trigger.
- If your water temperature is above 80 degrees Fahrenheit, keep the lights off until you can safely lower the temperature.
How to nurse a bleached coral back to health
A coral that bleaches is not dead yet. The stressed organism is now more vulnerable to infection and cannot meet its own nutritional needs; however, it is still possible to save it.
Did you identify and remove the stressor?
- Water Parameters: If your temperature, salinity, pH, or another water parameters change significantly, gradually return it to its ideal state through a series of water changes or supplementation.
- Water Contaminant: If your tank experienced rapid contamination, that calls for quick intervention, usually in the form of a series of significant water changes (~20% each) to remove the contaminant triggering the issue. If possible, relocate corals to another tank without impurity.
- Lighting: If you changed your lighting or changed the position of the coral under the lighting, you will want to dim your lights, move the coral back or partially shade the coral to provide relief from the light.
Keeping it alive long enough for the zooxanthellae population to regrow
- Meet the corals nutritional needs through direct feeding: Perhaps the most acute issue associated with coral bleaching is that the coral has lost the caloric contributions from the zooxanthellae. So your job over the next month is to meet all the nutritional needs of the coral through feedings. Providing the specimen with coral food by direct feeding it anywhere from one to three times per week will need to become a part of your routine. It will then be essential to keep an even closer eye on your water parameters, as frequent feedings could lead to a spike in nutrients.
- Provide adequate flow to minimize the chance of infection and algae growth: The indirect consequences of bleaching are that the coral cannot defend itself naturally from infection or algal growth. During this time, ensure the coral receives adequate water flow and immediately remove any encroaching algae.
Bleaching is a stress-induced event that leaves your corals vulnerable to starvation, infection, and tissue loss. With the proper care, you can reverse the impact of bleaching if you can remove the underlying issue and then nurse the colony back to health.
This article was written by Albert B. Ulrich III, author of The New Saltwater Aquarium Guide and Saltwater Aquarium Blog. If you enjoyed this piece and want to further your research into coral keeping, we recommend checking out some of Albert’s other articles on his blog, including some of our favorites listed below:
- Aquarium Corals: The Definitive Reef Tank Guide
- Saltwater Aquarium Setup: Definitive Step-By-Step Guide
About The Author
Albert Ulrich is a reef aquarium hobbyist, a renowned author, and a beloved husband and father of three. Albert has been the owner and primary gatekeeper of the Saltwater Aquarium Blog since 2009 and is responsible for attracting thousands of individuals to the reef aquarium hobby. In addition to his blog, Albert has also published four titles available for purchase on Amazon. Much of Albert’s content focuses on helping beginner reefers be successful and assisting already established hobbyists in improving and continuing the enjoyment of their reef aquariums. Albert enjoys spending time with family and friends when he isn’t writing or keeping up with his own aquariums.
Borneman, E., 2009. Aquarium corals. Charlotte, VT: Microcosm Ltd.