Understanding Coral Bleaching and Color
Tiny plant-like organisms called zooxanthellae live in the tissues of many stony corals. These microscopic algae capture sunlight and convert it into energy just like plants, to provide essential nutrients to the corals. “Bleaching” is actually not always when a coral itself changes color, it is caused by zooxanthellae dying and leaving the host caused by stress. Zooxanthellae are stressed by high temperatures, low water flow, excessive light or a combination of these conditions.
The zooxanthellae also provide much of the red, green, and brown colors that corals have. The less common Blue, purple and mauve colors found in some corals the coral makes itself.
The most likely factor impacting coral color is light intensity. If light intensity is less than optimal for a species the coral can appear darker brown. If the light is brighter than optimal the coral can have a lighter appearance.
Water quality is also important for nicely colored heathy coral. Maintaining proper levels of calcium, carbonate hardness and magnesium is a must, as are reasonably low levels of nitrate and phosphate. For those using methods other than frequent water changes to reduce nitrate and phosphate dosing trace elements will be more important. Dosing vitamins, amino acids and feeding can accelerate growth and help maintain nice color.