Coral bleaching is a phenomenon that occurs when coral polyps, the tiny animals that make up a coral reef, expel the algae that live inside their tissues. This can happen for a variety of reasons, but the most common cause out in the natural reef is a rise in water temperature. When the water gets too warm, the coral polyps become stressed and kick out the algae. Without the algae, the coral turns pale or white, what we refer to as "bleaching."
In your home aquarium things are a bit different. We are tasked with more than just controlling the water temperature. We are responsible for supplying the corals with EVERYTHING they need to survive. In the ocean things naturally stay more stable as a new supply of water is flowing over the corals every day. In the home aquarium it is far harder to keep levels stable. Some corals are more forgiving of our mistakes than others and some corals have very strict demands.
If your corals are bleaching the first thing you should do is think back on if you've made any changes or have added anything to the water within the last few days. Sometimes it takes a couple days for a coral health issue to progress to an extent that it is apparently visible. Here are some common causes of coral bleaching that we see in the aquarium hobby...
Lighting - Most often corals bleach from the par of your lighting being too much for the coral to handle. If this occurs the coral will eject it's zooxanthella in an attempt to regulate it's bioactivity. For this reason many reefers start their corals down low and gradually expose them to higher par until they are happy in the final location chosen by the hobbyist. This process may need to be stretched out over the course of a few weeks. Nutrients (nitrates) also play a role in how much light a coral can handle. Typically corals can benefit from higher par only if there are nitrates present in the water. A coral that was doing just fine in high par can begin to bleach if your nitrate levels drop too low.
Temperature - If you are the unlucky person who has discovered your aquarium heater has become stuck in the "on" position and your water is hot then you know how this can be devastating to your home reef aquarium. Depending on where you live, hot summer days can be enough to raise your tank's temperature to critical levels. We recommend a tank temperature of 75-78F and we set ours at 76F. Bleaching may occur somewhere around 85F and gets worse the higher you go. If you discover your tank is too hot we recommend you don't remove the corals unless the water is cloudy. Let the tank temp come down slowly on it's own and add sealed bags of ice to help if needed. It is important not to do anything too stressful and to make any changes as slow as possible.
Changing Alkalinity levels too quickly is a leading cause of coral bleaching or death. We recommend testing every batch of newly mixed salt water for Alk levels before doing a water change. Also if you are dosing for Alkalinity try to raise or lower it as slow as possible. We shoot for less than 1dKH per week if making adjustments. Also it is very important to never dose a water chemistry product if you can't or didn't test levels. There is no need for guesswork with all of the great affordable testing equipment that is available to us hobbyists.
Alkalinity is a measure of the ability of the water to neutralize acids, and it is important for coral health because it helps to maintain a stable pH level in the water. Coral polyps, the tiny animals that make up a coral reef, need a specific range of pH to survive and grow. If the pH level in the water becomes too low or too high, the coral can become stressed, which can lead to bleaching or death.
Alkalinity plays a key role in regulating the pH level in the water. If the alkalinity is too low, the water will be more susceptible to changes in pH, which can harm the coral. On the other hand, if the alkalinity is too high, the water will become more alkaline, and the coral will also be stressed. It is important to maintain the appropriate level of alkalinity to ensure that the pH level stays within the optimal range of 7-12 dKH for the coral and to limit changes to less than 0.2 dKH per day.
Toxins and Contaminants
There are other stresses on coral that can lead to bleaching. The "quick cures" sold online that promise fast results to combating common issues faced by aquarists can cause problems. Many of these can have adverse effects on coral health and should be used with caution or avoided altogether. If you are thinking of using a product to change something in your tank be sure to always follow directions and think of how it may effect other parameters.
Toxins can also be added to your water from certain corals or pests. Nepthea coral is known to secrete a toxic compound into the water that can be harmful to other corals. Flatworms are also very toxic when they die. Sea cucumbers have also been known to release toxins if they were to die in your tank. Check for issues and if you find an animal that has died take care to remove as much of it as possible.
Talk to Other Reefers
There are a ton of great resources out there for finding and diagnosing problems with your coral health. Join your local reefing club or group online and chat with fellow reefers about your problem. You'll find that there are a lot of people willing to give you advice. It's a good idea to weigh the advice based on the persons professionalism and years of experience. Advice that is given by many seasoned reefers should be considered more reliable but may not always be the best option. Do your research and remember that nothing good happens quickly. Go slow and try not to make big changes and your reef will reward you with great colors and growth.
If we have a coral that is bleaching then what do we do? First we inspect for "critters" or any nearby corals that might be stinging it. Then we clean, dip and move the coral to a lower par lighting area. If it's just one of your corals that is bleaching then moving it to a lower lighting area typically solves the problem. It may take weeks or months to regain it's color and dosing live zooxanthella may help the coral recover. If more corals are bleaching in different parts of your tank (that were otherwise doing fine previously) then we go right to checking nitrates. If they are reading zero then you can raise them with sodium nitrate or by feeding more often. Your corals may be starving and a little nutrition can make them happy.
If you have any other ideas to be added to this list, please let us know in the comments!
Brian Hale has enjoyed 25+ years of dedicated involvement in the reefing community. His commitment to research and extensive hands-on experience makes him an invaluable source of expertise within the reefing hobby.