RTN (Rapid Tissue Necrosis) is exactly what it sounds like - a coral colony loses tissue from the base or a single branch outward at a visible rate, sometimes within hours. STN (Slow Tissue Necrosis) is the same disease process running at a slower clock, often spreading over days to weeks. The exposed white skeleton is what you see; the dead coral is just the most obvious symptom. Both are typically triggered by an environmental stress event followed by bacterial infection that takes hold in the stressed tissue.
When you spot RTN/STN, action comes before investigation. Get the affected colony into a separate container with tank water and an air stone. Use a Dremel or coral saw to remove every section of skeleton with active tissue loss, cutting at least 1 cm into healthy tissue. The healthy fragments go into a frag tank or a separate quarantine container with strong flow. The diseased material is discarded. Skip dipping the original colony - dipping after a major loss adds stress to already-failing tissue.
Once the colony is fragged and stable, audit every parameter you can test. The majority of RTN events trace to one of: alkalinity swing greater than 1 dKH in 24 hours, salinity drift outside 1.024-1.027, temperature spike or crash, low oxygen at night, or an over-dose of a supplement. Pull a complete ICP test from Triton or ATI - many cases trace to a heavy-metal contamination (copper, aluminum, iron) that no hobbyist test kit detects. The colony was not failing for no reason.
Healthy fragments from an RTN colony go into low to moderate flow with moderate light for at least 2-3 weeks. Drop them in a frag rack 12-15 inches below your light source and resist the urge to move them. Encrustation on the cut sites is the first sign of survival - you should see polyp extension and color return within a week if the frag is going to live. Dose amino acids 2-3 times per week to support regrowth.
Stable alkalinity is the single biggest preventative measure for SPS-dominant tanks. Once you have more than three or four colonies, manual dosing is not enough - install a calcium reactor, a two-part doser, or a kalk reactor with consistent output. Quarantine and dip every new SPS specimen with CoralRx or Bayer Advanced Insect Killer (yes, that one - reef hobbyists have used it for years). Run a UV sterilizer if you have unexplained losses, since some bacterial agents implicated in RTN are killed by UV exposure.