Coral bleaching occurs when a coral loses its zooxanthellae (photosynthetic algae) leaving their tissue behind, transparent. The white appearance is the skeleton of the coral seen through the transparent tissue. Bleaching, loss of zooxanthellae from coral tissue, can occur due to these RTN and STN parasites as well as other factors such as poor lighting or prolong elevated water temperature. In an aquarium with stable parameters, bleaching is most likely due to early coral tissue infection with microscopic RTN and STN parasites. If left untreated, the bleached areas will necrosis to expose skeleton. Early treatment of a bleaching coral with Prime Coral STOP RTN dip and Prime Coral Prevent in-tank treatment is highly advisable as they are the only know treatments for these infectious parasites.