RTN and STN: What to Do in the First Hour

RTN and STN require action within the first hour. Fragging below the visible necrosis line, isolating the cut piece, and checking your water parameters immediately are the three steps most likely to save the coral and protect the rest of your tank.

What RTN and STN actually are

RTN (Rapid Tissue Necrosis) and STN (Slow Tissue Necrosis) both describe the same core process: coral tissue dying and sloughing off the skeleton, leaving bare white bone behind. The difference is pace. RTN moves within hours, fast enough that you can sometimes watch it progressing across a colony in real time. STN unfolds over days, which can make it easy to dismiss early on as bleaching or a minor stress response.

Neither is a disease in the traditional sense of a single pathogen invading a healthy animal. Both are typically the end result of an underlying stress event that pushed the coral past its tolerance threshold. The tissue loss is the symptom, not the cause.

Why the spread matters so much

Left unchecked, RTN and STN can spread to adjacent corals, not just continue across the original colony. That is what makes the first hour so critical. Every minute of hesitation is time the necrosis front has to advance, and time bacteria and opportunistic pathogens have to colonize newly exposed skeleton and potentially transfer to neighboring tissue.

How to recognize RTN versus STN before you act

The visual signature is similar in both cases: a clean, receding tissue line exposing bright white skeleton. Healthy tissue sits on one side; bare bone on the other. There is no gradual fade or patchy browning the way you see with bleaching.

With RTN, the line moves noticeably during observation. If you check a coral, look away for twenty minutes, and come back to find the bare patch has grown, you are dealing with RTN. With STN, the line advances slowly enough that you might not notice the change without comparing photos taken a day apart.

If multiple corals in the tank are showing tissue recession at the same time, that is a strong signal pointing toward a system-wide event rather than a single specimen's individual reaction.

What to do in the first hour of RTN or STN

This is the sequence that gives you the best chance of saving tissue and protecting the rest of the tank. Do not wait to see if it stabilizes. Stabilization without intervention is rare, and the cost of waiting is almost always losing more coral.

Step 1: Frag immediately, well below the necrosis line

Pick up a sterile cutting tool and frag the coral below the visible necrosis line, cutting into apparently healthy tissue. The goal is to get ahead of the recession front. If you cut right at the edge of visible tissue loss, the front may already extend microscopically beyond what you can see, and you will find yourself cutting again within the hour.

Farther is better. A smaller frag that survives is worth more than a larger piece that continues to recede. Use a clean, sterile tool to avoid introducing additional stress or contamination to the cut surface.

Step 2: Move the frag to a separate, stable system

Once you have a cut frag, it needs to go somewhere other than the display tank. A dedicated quarantine or frag system is ideal. At minimum, move it to a different flow zone where it is not sharing direct water column contact with the corals that may have contributed to or been affected by the triggering event.

The logic here is straightforward: whatever stressed the original coral is still present in the main system. Keeping the frag there means continuing to expose it to the same conditions that caused the problem. A separate, stable environment removes that variable and gives the frag the best chance of recovering and holding tissue.

Step 3: Check water parameters immediately

Once the frag is isolated, turn your attention to the main tank and pull a full parameter check. RTN and STN frequently follow a parameter swing in the prior day or two. A temperature spike, an alkalinity crash, or a contamination event are common triggers. The coral may have been under stress for hours before the tissue loss became visible.

Look at your recent parameter history if you have it recorded. A swing you noted and thought the tank recovered from may have been the event that set this coral's necrosis in motion. If you do not have a history to review, this is the moment to start keeping one.

What to avoid during an RTN or STN event

A few common mistakes tend to make outcomes worse:

• Waiting to see if it stabilizes. The phrase "let's see what happens overnight" almost always results in losing the whole colony by morning when RTN is involved.
• Cutting at the visible line rather than below it. This leaves you right at the advancing front. Cut further into healthy tissue.
• Returning the frag to the main tank too soon. The trigger event may still be present. Keep the frag isolated until you have identified and resolved the underlying cause.
• Assuming it is bad luck. A single coral going RTN in isolation can reflect that specimen's individual sensitivity to a stress it handled worse than others. But multiple corals showing RTN or STN around the same time is not coincidence. That pattern points to a system-wide event and deserves a thorough investigation.

Investigating the trigger after the emergency response

Once the immediate crisis is handled, the investigation matters as much as the intervention. RTN and STN are rarely random. They follow something: a temperature spike from a failed chiller, an alkalinity crash from a depleted dosing system, a contamination from a new product or livestock introduction, or a large water change with mismatched parameters.

Review the prior 24 to 48 hours carefully. What changed? What did you add, adjust, or neglect? If more than one coral was affected, the trigger is almost certainly systemic, and finding it protects everything else in the tank.

If your parameters look stable on the surface, consider less obvious sources: a dead fish or invertebrate decaying in a sump or behind a rock, a newly introduced coral or frag carrying a pathogen, or a piece of equipment leaching something into the water column.

Tracking parameters consistently over time makes this investigation much faster. When you have a log to reference, you can often spot the swing within minutes rather than guessing.

How to prevent RTN and STN going forward

Because both conditions are typically triggered by stress rather than arising spontaneously, prevention comes down to stability and monitoring.

• Keep parameters stable rather than chasing ideals. A stable reading that is slightly off is generally less harmful than a perfect reading with frequent swings.
• Monitor temperature, alkalinity, and salinity consistently, since these are the parameters most commonly implicated in stress events that precede tissue necrosis.
• Quarantine new coral before adding it to the display tank, giving you a chance to observe it under stable conditions without risking the main system.
• Keep a parameter log. Whether you use a notebook, a spreadsheet, or a dedicated app, a running history of your tank's chemistry is the single most useful diagnostic tool when something goes wrong.

Frequently asked questions

Can a coral fully recover from RTN or STN if you frag it in time?

Yes. A frag cut below the active necrosis line and moved to a stable, separate system can hold tissue and recover. The key variables are how far ahead of the front you cut and how quickly you remove the frag from the stressor that triggered the event in the first place.

Does RTN always spread to other corals in the tank?

Not always, but the risk is real enough that isolation is the standard response rather than a precaution reserved for severe cases. Adjacent corals sharing the same water column and flow patterns are the most vulnerable.

Is there a dip or treatment that stops RTN in progress?

The original post does not describe specific dip protocols or medications, and introducing unverified treatments during an active event carries its own risks. Fragging below the necrosis line and isolating the frag in a stable system is the intervention with the clearest rationale.

How do I know if it is RTN or STN, and does it change what I do?

The first-hour response is the same regardless of which you are dealing with. Frag, isolate, and check parameters. The pace of progression will become clearer over the following hours, but waiting for confirmation before acting costs tissue you will not get back.

What if I do not have a separate tank to isolate the frag?

A separate tank is the best option, but if one is not available, move the frag to a different flow zone in the main tank as a short-term measure while you investigate the trigger. The goal is to reduce its exposure to whatever is driving the event and to separate it from neighboring corals that could be affected by spread.

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TankMind can help you log parameters, track coral health events, and build the kind of history that makes diagnosing a future RTN or STN event much faster.