This is confused by many SPS keepers. You'll often read that an SPS coral has bleached, when it has actually died. A bleached coral will still have its tissue, however, the zooxanthelle living within the coral will expel causing it to lose it's color. The severity of bleaching can vary. A coral can be slightly faded in color, or completely white (but still alive) for a variety of reasons. You can usually see polyps on the coral.
Bleached but alive
Tissue necrosis is placed into 2 categories, Rapid Tissue Necrosis (RTN), and Slow Tissue Necrosis (STN). When either occurs, a coral is losing it's flesh. RTN will usually show a "peeling" of the tissue, and death occurs usually in less than a day.
Here's a pic from Sanjay's Tank showing RTN
STN usually occurs from the base of an SPS coral. This could happen for days or months before it consumes a coral. Sometimes it will stop and the coral will regrow over the dead area.
An example of STN
Bleached but alive
Tissue necrosis is placed into 2 categories, Rapid Tissue Necrosis (RTN), and Slow Tissue Necrosis (STN). When either occurs, a coral is losing it's flesh. RTN will usually show a "peeling" of the tissue, and death occurs usually in less than a day.
Here's a pic from Sanjay's Tank showing RTN
STN usually occurs from the base of an SPS coral. This could happen for days or months before it consumes a coral. Sometimes it will stop and the coral will regrow over the dead area.
An example of STN
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