RTN and STN in SPS Corals

What it looks like

Bulk Reef Supply describes both as visible tissue loss with a clear line between living tissue and bare skeleton. The only difference is speed: fast is RTN, slow is STN. A brown, jelly-like film on the dying tissue is also common.

Bleaching is different: a bleached coral is pale but still has its tissue. See coral turning white.

What causes it?

Bulk Reef Supply is clear that no one knows the root cause. It reports suspects rather than answers: bacteria linked to tissue damage in the wild when water warms, ciliates found on affected corals, and physical damage from handling, stinging neighbours or pests such as flatworms.

Many reef keepers also link it to sudden changes, especially in alkalinity and temperature. That isn’t proven, but keeping conditions steady costs little and rules out the obvious.

What to do now

Act quickly; RTN can move fast.

• Frag above the line. Bulk Reef Supply suggests cutting at least 1.25 cm (½ inch) ahead of the dying edge, and discarding the affected piece.
• Siphon dead tissue out of the tank if you can.
• Consider a coral dip for the healthy frags; products vary, so follow the label.
• Check the basics: temperature, alkalinity, salinity. Compare with your log.
• Look for pests on other colonies with a torch.

Keep things steady afterwards

Keep alkalinity and temperature as steady as you can, and avoid big changes while the tank recovers. Log tests every few days for a while, so you’d spot a swing if one happened.

Is RTN contagious?

It can spread to other colonies, but it’s not clear why. Remove affected pieces promptly.

Do alkalinity swings cause RTN?

Many reef keepers link them, but it isn’t proven. Steady alkalinity is sensible either way.

Can I save an RTN coral?

Often only by fragging the healthy part well above the dying edge.