Diagnosing a Sick Coral: Bleaching, STN, and RTN

When a coral starts to decline, time matters, and so does an accurate diagnosis. Corals show distress in a few characteristic ways, bleaching, slow tissue necrosis, and rapid tissue necrosis, and telling them apart guides how urgently and how you should respond. A bleaching coral and a coral suffering rapid tissue loss need very different reactions, and confusing them can cost you the colony or even its neighbors. Learning to read these warning signs quickly is a valuable skill for any reefer keeping stony corals. This guide explains how to diagnose a sick coral, distinguishing bleaching, STN, and RTN, and what to do about each.

Recognizing Bleaching

Bleaching is the loss of a coral’s color as it expels its symbiotic algae, turning pale or stark white while the coral tissue itself remains present on the skeleton. It is the coral’s stress response to adverse conditions, commonly elevated temperature, but also intense light changes, parameter swings, or other stress. A bleached coral is not necessarily dead; if conditions are corrected, it can regain its algae and recover, though a severely or prolonged bleached coral may not. The key sign is that the tissue is still there but colorless. Bleaching is a warning that something in the environment has stressed the coral, and it calls for finding and fixing that stressor.

Slow Tissue Necrosis

Slow tissue necrosis, or STN, is the gradual death of coral tissue, typically progressing from the base upward over days, leaving exposed white skeleton behind as the living tissue recedes. Unlike bleaching, where colorless tissue remains, STN means the tissue is actually dying and peeling away, revealing bare skeleton. Because it advances slowly, STN gives you some time to respond, but it is serious and will consume the colony if unchecked. It often stems from chronic stress, instability, poor water quality, or disease. Identifying STN means noticing the creeping line of exposed skeleton advancing across the coral, and it signals a need to investigate and correct underlying conditions before it spreads further.

Rapid Tissue Necrosis

Rapid tissue necrosis, or RTN, is the emergency of the three. It is the same tissue death as STN but dramatically faster, sometimes stripping a coral of all its tissue within hours, leaving bare white skeleton with terrifying speed. RTN is most associated with SPS corals and can be triggered by acute stress, severe parameter swings, or infection, and it can sometimes spread to nearby corals. Because it moves so fast, RTN demands immediate action; there is no time to wait and watch. Recognizing RTN, tissue sloughing off rapidly rather than receding slowly, and reacting at once is critical to saving any of the coral, and to protecting its neighbors.

Responding to Each

Your response depends on the diagnosis. For bleaching, find and correct the stressor, often temperature or a parameter swing, and stabilize conditions so the coral can recover its algae; avoid further shocks. For STN, investigate and correct the underlying instability or water quality issue driving it, and stabilize the tank. For RTN, act immediately: the common emergency measure is to frag off the healthy, unaffected tissue, cutting well ahead of the receding line, to salvage a portion of the coral before the necrosis consumes it all, and to remove the dying coral from contact with others. In every case, restoring stable, clean conditions is the foundation of recovery and prevention.

Building a Coral Triage Routine

When a coral starts to decline, the speed and order of your response matters enormously, and having a triage routine ready turns a moment of panic into effective action. The first step, before touching anything, is to check your water parameters, because a sudden problem across one or more corals often traces back to a swing in salinity, alkalinity, or temperature. Confirming whether the chemistry is stable tells you immediately whether you are dealing with a tank-wide cause or an issue isolated to one coral.

Isolating the affected coral is frequently the next move, especially with rapid tissue necrosis. Removing a coral that is losing tissue quickly from the display protects its neighbors, since the conditions or pathogens involved in a fast decline can affect nearby corals. Moving it to a separate container or frag tank where you can observe and treat it closely, without risking the rest of the reef, buys you control over a situation that can otherwise spread.

For necrosis that is advancing, fragging ahead of the dying tissue is the classic rescue. Cutting off the still-healthy portion of the coral well beyond the line of receding or dead tissue, and remounting that healthy frag, preserves living coral before the necrosis consumes it. With rapid tissue necrosis in particular, acting fast and cutting generously into healthy tissue gives the saved frag its best chance, since hesitation often means losing the whole colony.

A coral dip and clean, stable conditions support whatever you save. Dipping the affected or fragged coral can help guard against secondary infection and dislodge any pests contributing to the problem, while placing the survivor in steady, clean water gives it the calm environment it needs to stabilize. Resisting the urge to make further drastic changes, and instead holding conditions rock-steady, lets a stressed coral recover rather than facing a second shock.

Throughout triage, observation guides every decision. Watching how the coral responds over the following hours and days, whether the tissue loss halts or continues, tells you if your intervention worked or if more aggressive fragging is needed. Keeping notes on what you saw, what you did, and the outcome builds your judgment for next time. A calm, practiced triage routine, applied promptly, is what most often turns a sick coral into a saved one rather than a loss.

Diagnose Fast, Act Right

Telling bleaching, STN, and RTN apart lets you respond correctly when a coral declines. Bleaching leaves pale but present tissue and calls for fixing the stressor; STN slowly exposes skeleton and demands you correct underlying instability; RTN strips tissue rapidly and is an emergency that may require fragging healthy tissue at once. In all three, stable, pristine conditions are both the cure and the prevention. Learn to read these signs quickly, act according to the diagnosis, and you give your corals the best chance to recover.

Related reading

• Reef Tank Lighting Explained: PAR, Spectrum, and Photoperiod for Coral Growth
• Coral Aggression: Sweeper Tentacles and Chemical Warfare
• Aquascaping a Reef Tank: Stability, Flow, and Coral Real Estate
• Mushroom Corals: The Bulletproof Beginner Coral

Sources & further reading

• NOAA Coral Reef Conservation
• National Aquarium

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