What Causes Coral Bleaching?

The keeper of an established and carefully maintained reef aquarium usually does not need to be concerned about coral bleaching. The corals have been in situ for a lengthy period and are well settled. A change in environment can upset this idyllic situation.

Bleaching is popularly linked to hard corals, but is applicable to soft corals also.* ‘Bleaching’ is a term given to describe a particular situation, and the term is pretty accurate. Many, possibly the majority of corals, harbour symbiotic algae, known as zooxanthellae. This algae is of benefit to the coral. For several reasons the coral may expel the zooxanthellae, leaving the coral looking white or jelly white. Anything from 50 to 90% of the zooxanthellae is ejected.* The coral is still alive, it has lost the colouring algae within.

As said, there are several reasons why a coral may bleach, and the following demonstrates

Bacteria, chemicals, ciliates, coccidians, darkness, fungi, heavy metals, higher
temperature, hypersalinity, hyposalinity, lack of water movement, light,
medications, noxious agents, physical stress, red spectral light, sedimentation,
starvation, stress, temperature change, and increased ultraviolet radiation.*

Reading that lot could make an aquarist wonder if it is worth starting a reef in the first place. Well, it isn’t as bad as it sounds. It is very much like reading a gardening book – the pests and diseases that can afflict a plant species are a long list.

If the aquarist is careful in selection and placement in the aquarium, minimising handling, then stress is minimised. Stress is not only created by handling though. If there are continuing fairly large fluctuations in temperature, and/or the specific gravity similarly fluctuates, then the corals are going to be stressed. Stability should be maintained. The water should not be full of sediment, if it is there is something amiss with the set-up and the husbandry – it needs correcting. Sediment can settle on a coral, reduce light, and cause stress. No aquarist is going to put anything in the aquarium that is dangerous to corals, for example metals, insecticides etc, and if reverse osmosis water is used then this danger is minimised. Oxygenation of the seawater is very important, if there is plenty of oxygen then the water movement could be adequate, but note that the guideline for movement in a soft coral display tank is 10 times per hour, and in a hard coral display 20 times or more an hour. If not, it needs correcting. A lack of light can cause stress, the algae need light and a lack will cause problems. The light should be of adequate power (watts) and suitable spectrum. Metal halides are the normal lights used for hard corals. They emit ultraviolet light but the bulb or canopy normally includes a UV filter. This doesn’t cover everything in the list but all that is probably necessary.

It follows that if a well designed reef system is stable then all should be well. The aquarist should not suddenly increase or decrease the temperature because it is thought a different temperature is better, it should be achieved slowly over time. Likewise specific gravity. The power (watts) of metal halide lighting should not be suddenly increased. If there is a reason for doing so, then the output from the lights should be controlled by raising the bulbs further away from the water surface, and over a period of time slowly                        lower them. Fluorescent lighting changes should not be as problematical, but watch for spectrum changes in the tube output, and whatever the lighting, watch the corals.

Corals as part of their normal life eject zooxanthellae* and the aquarist will not usually notice. These ejections are because the algae is becoming too numerous, or it is old and is being replaced.

Though there is argument about whether wild bleaching is a natural cycle or not, large bleaching events take place on coral reefs. Bearing in mind the list of possible causes, some of it is probably the result of pollution such as sedimentation, heavy metals, insecticides etc that are in land run-off.

The aquarist may hopefully never see bleaching. In all the years I have kept soft corals under fluorescent lighting, it has never occurred. Nor has it with Peter and his hard corals, lit by metal halides. Do maintain the system properly, then simply enjoy.

(* Reference: Eric H. Borneman. Aquarium Corals)

  1. this is more of a question. For about 3 days my green starburst polyp was doing good. Then for some reason it flopped over and now the tips are turning white; what could be the problem? Are anitic lights to strong; should i be using a daylight bulb? Help!

  2. Hi Rob.
    Difficult to know why, generally.
    Actinic lights don’t cause corals to collapse. They are targetted at the symbiotic alge within corals, as science tells us that this light spectrum is what is required. They do make the aquarium attractive though don’t they!
    You should not be using actinic lighting on its own. The normal practice is to mix actinic with marine daylight. If you are using tubes then it is normal to match one actinic for one daylight. Many metal halide users have actinic tubes in use. The use of reflectors is recommended.
    I use tubes as described. The actinics come on first but I have noticed that the corals seem to react (expand) to the marine whites the most.
    Remember also to check your seawater quality – is it up to the mark?

  3. This is a question regarding an anemone. Mine has turned hard and is white on the sides. It still has its color on the top. I wasn’t certian it was dead so I contacted an aquarium store and they seemed to think it was a bleaching problem. Could this be? Should I seperate it from the tank?

  4. Hello Heath.
    Anemones are not the easiest to keep in an aquarium unlike what some think. Once settled they are long lived and entertaining, particularly with a clownfish (if it is of that type).
    Generally, anemones have symbiotic algae in their flesh. This means they nust have the proper lighting, the same as is used for corals. It sounds as though the algae is being lost. I suggest you check the lighting. They also need high water quality and good water movement, again like corals. I don’t really understand why it is going ‘hard’.
    It just so happens that an article on anemones called “Clownfish Anemones” has been put on the Blog. It went on on 21 April. A read may be of help.
    Why not visit the Forum, where questions on all sorts of topics can be placed and answered. The forum is in the social area:

  5. my all hard corals ar dying
    every parameter in my tank is ok
    can you help in to

  6. Hello.

    If all your seawater parameters are up to standard then there are two areas that come to mind.

    The first is seawater movement, is it adequate? Generally, hard corals require more movement than soft types. Have a look in ‘Articles’ on this website for more information on the subject.

    Second and perhaps more likely is lighting. Generally hard corals require stronger lighting than soft corals. The depth of the aquarium also comes into the equation. Some aquarists use metal halide, others use fluorescent tubes, and some use LED arrays. Even if the lighting is of the correct power and spectrum, bulbs need to be changed on a regular basis as they lose efficiency over a fairly short period (this does not apply as much to LED systems). Again, perhaps a look in ‘Articles’ would be of use.

    I hope everything comes right.

  7. I have a couple problems in my 120gal tank.My Capricornis,which were green and orange approx. 10 in.in diametor started getting a gel on the edges which kept getting larger.I diagnosed it as brown jelly diease and started dosing with Sodium Ascorbate.My dosage started at 5ppm and worked up to 50 ppm after 4 days .I was going to stay at 50 for 10 days but noticed my brown Acropora starting to bleach,so I stopped the Sodium Ascorbate dosing after 8 days .My halide bulb was just relaced with a 250W 14000k bulb.The same type I had before .My old bulb was a year and a half old.Is the bleaching caused from the new bulb or the dosing? I am thinking the Sodium Ascorbate made them more sensitive to the lighting .I use reverse Osmosis water,my numbers are good and I have plenty of movement in the tank. I would appreciate any help you can offer Thanks Jim

  8. Hello Jim.
    I’m sorry to read of your problem.
    Montipora Capricornis, a small polyped stony coral, is popular and once established in the aquarium is reasonably hardy (in SPS terms). For continued health it’s important that nitrates are low (10ppm or less is the guideline) and phosphates as low as possible (undetectable). Seawater movement should be adequate, the guideline is 20 times the net gallonage of the display aquarium per hour or more. All your readings and movement are reported as good.
    I wouldn’t think the lighting has caused the problem. Metal halide and T5 tubes are both suitable, though depth of aquarium is more critical with T5’s. The coral can be successfully kept under both. I’m not sure about the increased sensitivity of the coral but as you have been using a medication this could be the case.
    It is possible that the disease, which is bacterial, has been caused by damage. This could have been you inadvertently catching it and causing some damage, a crab……it goes on.
    The jelly could possibly be siphoned out but clearly care is needed with this. All of it can’t be removed this way and this will not cure the disease problem.
    There are ways of tackling the disease and it is best to direct you to a website rather than type a great deal. Perhaps you would like to visit http://www.athiel.com/lib/bacterial.html
    I hope all goes well.

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