There are organisms called zooxanthellae (microscopic algae) which live inside the tissue of many corals. This is demonstrative of a symbiotic relationship which benefits both creatures. It is these microscopic algae’s photosynthesizing behaviour that gives the corals their colours. When the corals are subjected to various stresses, they tend to expel the zooxanthellae thereby exposing the coral’s white skeleton. The loss of colour by corals places them at risk and it is better to prevent this, than to try coping with it.
Most corals are not able to survive without zooxanthellae, as they struggle to feed themselves and will eventually starve. Corals will lose colour if subjected to some stress. The major known cause of colour loss in corals is continued temperature increases mainly due to global warming. The corals turn whitish (coral bleaching) and eventually die. Temperature increases in your aquarium can have the same effect.
Other pressures like changes in chemical water composition can also contribute to your coral’s loss of colour. Once you notice that your corals are losing colour, it is very important that you do water quality tests and take appropriate corrective measures. Too much nitrite, ammonia or phosphates can subject your corals to stress resulting in their loss of colour and eventually their deaths.
It is encouraged that you do a 30% to 40% water change from your aquarium once you find out that your corals are losing colour. Also try to maintain adequate lighting in your aquarium. A decline in lighting can exert pressure and stress to your corals and they can lose colour.
You must frequently check your water’s pH level. Avoid habits like overfeeding your aquarium as this can promote increased ammonia due to decomposition. Also strive to remove dead fish and corals from your aquarium. Make sure you do not expose your aquarium to bad smells, pesticides, herbicides and insecticides and even perfumes. If these are sprayed near the aquarium tank, they can diffuse into the tank placing your corals in danger.
A key important issue is spacing. Corals need space. Overcrowding them will lead to stress as they compete for life sustaining nutrients, sunlight, and oxygen. The chances of corals losing colour are greater if your corals are overcrowded. If you still think all your aquarium parameters are normal but your corals continue to suffer colour loss, then immediately consult a specialist in aquarium health.