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)