Many people have had corals bleach, whether soft, SPS
, or LPS
. Coral bleaching is basically a purging of the symbiotic zooanthellae algae from inside the coral polyps. Zooanthellae captures sunlight (or in our hobby, artificial light) and generates energy from it, in the form of photosynthate, and the coral utilizes that photosynthate as energy. Some corals use it as it's only source of energy, some supplement it by feeding, but all corals have zooanthellae. In return, the zooanthellae get the nitrate, phosphate, and other things it needs that are produced as waste by the coral.
When putting a coral into our tanks, we often acclimate it, often using drip acclimation or other methods of getting it used to your water chemistry. Typically, the corals are then placed in a pleasing location with a best guess about how much light they require. Little thought about acclimation to light is given and this is typically what causes most, but by no means all, cases of bleaching.
When a coral is suddenly exposed to more light than it is used to, and note that I didn't say more light than it needs, the zooanthellae begin producing more photosynthate than the coral can handle, it's something akin to overloading a nuclear reactor. To try and regulate the influx of energy that it can't handle, it begins to release zooanthellae from it's body to slow the production of photosynthate, which in the nuclear reactor analogy is like inserting the control rods to slow the reaction. If the coral is slowly acclimated to the lighting in your tank, it slowly releases a little zooanthellae at a time until it reaches the desired level, which is putting the control rods in slowly. When it isn't acclimated and is put under full tank lights, it goes into emergency mode and dumps as much zooanthellae as it can, which is like slamming the control rods in.
Now, typically, it can never get rid of *all* the zooanthellae by purging, so if the lighting is reduced again, they can begin to slowly reproduce and repopulate the coral, bringing it's color back and saving it. If lighting levels are left high, the corals can continue to be in panic mode and keep purging the algae as fast as it grows.
So how can we acclimate a coral? Several methods are available. One of the most common in hobby reef keeping is to place several mesh screens, like you'd use on a window or a door, over the top of the tank between the light and the water. By layering them they block a substantial amount of light, shading the coral. Every few days you take one screen off, allowing a little more light to penetrate into the water.
A second method is used by people with pendant lighting or other forms of fixtures hanging above the tanks but not rigidly mounted in a hood/canopy, including public aquariums. Light has a property whereby the amount of light that reaches a surface is reduced as a square to the distance. What that means is if a light is 1" from the surface of the water, and there is 100 lumens reaching the surface, by doubling the distance to 2" from the water, you reduce the lumens to 25. So doubling the distance equates to a quarter of the light, tripling the distance results in 1/9th the light, halving the distance results in 4x the light, etc. Using this fact, by raising the pendants, usually hung on chains, up a little bit, they can reduce the light quite a bit. The lights are then lowered a given amount, like one chain link, every day or two, until they are hanging at their normal height.
A third method is usable if you can manage it without impacting the health of the other occupants of your tank. Using a dimmer, unplugging some of the fixtures, or removing some of the bulbs, you can reduce the overall light reaching the tank. So then by slowly putting them back in service one light at a time, you are slowly increasing the lighting in the tank.
Using any of these methods will likely reduce the chances of coral bleaching, but will not eliminate them as there are other issues including stressful handling and disease which can cause corals to bleach.
Above I also mentioned that bleaching typically occurs when the coral experiences more light than it's used to but not necessarily more than it needs. If a coral requires 10000 lumen (all lumen values are hypothetical and do not relate to any actual coral for this example), but was stored at the LFS in a tank with 5000 lumen, and then goes into your tank at 8000 lumen, it is experiencing an increase that could cause bleaching, but is still well below the 10000 lumen it needs to be healthy. For this reason it should not be assumed that bleaching means the coral is getting more light than it needs to survive. It just means it's getting more than it has adjusted to in recent history.
I hope this article is helpful to those of you experiencing coral bleaching for the first, or even the fiftieth time. It's by no means an all inclusive explanation of coral bleaching, nor a written in stone guide to avoiding it, and I'm certainly not an expert by any means so if anyone else has any insight they'd like to add it's more than welcome.