Coral Bleaching

Coral Bleaching is the process in which your coral begins to expel its Zooxanthellae, this leaves the coral looking bone white and dead.  The reason that the coral does this is because the necessary conditions for your coral to host Zooxanthellae are not being met.  If you haven’t already noticed your coral is stressed before this point, take this as the last screaming sign that your coral needs help.  In almost all cases if some type of immediate action is not taken to save your coral from bleaching out, it may result in its death.

There are several causes of coral bleaching but the main ones for a marine hobbyist to focus on are;

  1. Buildup of nitrates, nitrites, phosphates or other harmful contaminants in your tanks water.
  2. Temperature swings in your tank
  3. Increase or decrease in light levels
  4. Increase or decrease in your water salinity
  5. Running bio-pellets and alkalinity gets to high (this has been termed as “Alk Burn”)
  6. Pest infestation
  7. Aggression from other corals (allelopathy or “chemical warefare”)


As a marine hobbyist the stability of your saltwater tank is key to having happy and healthy corals.  When one of the above factors changes too quickly coral bleaching will occur, and it can happen very quickly.  In several cases I have seen tanks crash over night from one of the problems listed above.

In this article im going to focus on a few corals which began to bleach from too much light.  In later articles I will address saving your corals from some of the other issues above.

Coral bleaching from light can happen in a few different ways.

The most common would be during bulb replacement.  Most reef aquariums use metal halide bulbs or T-5 bulbs.  The average life span of a metal halide bulb is one year and the life span of a T5 bulb is 18 months.  From the moment you first use your bulbs they begin getting weaker and slowing shifting spectrums.  Over the course of a year or 18 months the bulbs you originally had will be very different to the corals from what they were when they were brand new.  Simply changing the old bulbs to new bulbs and thinking your tank can continue to function as normal is sometimes a mistake especially if you have a lot of corals high up in your tank.  I would recommend on all bulb changes to toss a layer of window screen on top of your tank for a week or two until your corals can adjust to the new bulbs. If you notice your corals are beginning to bleach place an additional layer of window screen in between your lights and the top of your tank.  Also you can dial back the amount of time your lights are on per day to give the corals a longer dark period but I wouldn’t go any less than eight hours a day.

The opposite to this would be the corals not getting enough light, which the answer is simply to give them a longer time under the lights.  It is recommended to have your corals exposed to the light for 10 to 12 hours a day although I personally wouldn’t do more than 12.

Switching types of lights can also cause your corals to bleach.

The corals above were moved from a frag tank into a display tank.  The frag tank was running 35 watt T-5 bulbs and the display tank has three 400 watt metal halide bulbs.   A huge difference in light and even though these corals were set on the bottom of the display tank it wasn’t enough to keep them from bleaching out.  The bleaching of these corals happened over a period of two days.  If possible I would recommend that the corals be moved back into the frag tank they were taken from to heal.  In this case however this was not an option.  The next step in saving these corals was to place something between the tank and the light to shade the coral until it can heal and adapt to the new light.  Placing a layer of egg crate with two layers of window screen on top of the tank should do the trick.

When switching lighting systems say from metal halides to T5’s coral bleaching can also occur from lack of light.  A coral placed on the bottom of a tank with metal halides may have to be moved up in the tank to be closer to the T5 light to keep getting the same amount of light it was with the MH.


Just because your coral is bleached doesn’t mean it’s dead!  Many marine hobbyist would look at the corals above and write them off as dead and toss them from the tank.  The pictures of these corals are only a day or two after the bleaching event (03/22/2012) so I cannot post before and after pictures yet but as they heal I will revisit this post and hopefully be able to show some fully healed and happy corals.  I can say pretty confidently that the corals where you still see some color will bounce back just fine which would be the Birdsnest in the front left and the purple SPS in the back. The Tricolor SPS and the Catspaw SPSare iffy but I still have hope.

I will update this article in a few weeks with the results.



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