STN (slow tissue necrosis) and RTN (rapid tissue necrosis) are common SPS problems. Causes vary; water instability is a frequent trigger.
Check these first
- Alkalinity stability and recent swings (alk swings)
- Nitrate and phosphate near zero or sudden drops (balance)
- Temperature spikes
- Salinity changes
- Pests (AEFW, red bugs) and physical damage
Immediate actions
- Frag healthy tissue above the necrosis line and mount it.
- Stabilize alk and nutrients; avoid sudden corrections.
- Inspect for pests; dip affected corals.
- Run fresh carbon if you suspect contamination.
More on disease and pests: pests and diseases.
FAQ
Does RTN mean my tank is crashing? Not always. It can be a single colony issue, but check parameters and pests.
Should I raise alk to fix STN? Not quickly. Stability matters more than the number.