RTN and STN: What to Do in the First Hour

What you're dealing with

RTN (Rapid Tissue Necrosis) and STN (Slow Tissue Necrosis) describe coral tissue dying and sloughing off the skeleton — RTN within hours, STN over days. Both can spread to adjacent coral if not isolated quickly, and both are often triggered by an underlying stress event rather than a primary infection.

The first hour matters most

1. Frag immediately, well below the visible necrosis line. Cutting into apparently healthy tissue with a sterile tool can stop the spread — waiting "to see if it stabilizes" often means losing the whole colony.
2. Move the cut frag to a separate, stable system (or at minimum, a different flow zone) — isolate it from whatever triggered the event in the main tank.
3. Check water parameters immediately — RTN/STN often follows a parameter swing (temperature spike, alkalinity crash, a contamination event) within the prior 24–48 hours.

After the emergency response

Investigate the trigger rather than assuming bad luck — a single coral going RTN in isolation can be that specimen's individual sensitivity, but multiple corals hitting RTN/STN around the same time points to a system-wide event worth tracking down through your parameter history.