RTN and STN: Recognising and Treating Tissue Loss

RTN and STN: Recognising and Treating Tissue Loss

Few things trigger the same panic in an SPS keeper as a bare white patch spotted one morning at the base of an Acropora that was healthy yesterday. Tissue loss is the most feared problem in stony corals because it often progresses faster than you can react — at worst the whole coral is a bare skeleton within a day. The phenomenon affects SPS corals above all, but the same bacterial process can strike LPS corals too.

Two things decide the outcome: how fast you act and whether you find the root cause. Removing the symptom alone saves at most a single frag; without correcting the root cause the loss returns. This article covers what happens inside the coral, how to distinguish tissue loss from other causes, what triggers it, what to do in the first hours, and how the best-documented treatment — the Witchhazel tank treatment — proceeds step by step.

What happens during tissue loss

Tissue loss is not a single disease but an end result that several routes can lead to. What they share is that the coral’s own defence fails.

A healthy coral is not just an animal but a holobiont: the coral, its symbiotic zooxanthellae and the bacterial community living in its tissue form a whole. The coral produces antibacterial compounds in its tissue layer and maintains a beneficial microbiome that keeps the pathogens in the surrounding water at bay. As long as the coral is healthy and its calcium budget, nutrition and trace elements are in order, this defence works.

The problem arises when the balance tips in the pathogens’ favour. Either the coral weakens — for example through an alkalinity spike, heat stress or a trace-element deficiency — and can no longer produce its defensive compounds, or a pathogenic bacterium multiplies faster than the coral can suppress it. The usual culprit is a Vibrio-genus bacterium, which accelerates in warmth and low pH. Peer-reviewed research has additionally linked fast-progressing tissue loss to ciliates (protozoa), the same ones that appear in white syndrome — they may not be the original cause, but they feed on weakened tissue and accelerate the destruction.

Once this process begins, the tissue detaches from the skeleton and a bare calcium surface is left behind. The speed depends on which bacterial or protozoan strain dominates the tank and how weak the coral is.

Deeper on this: SPS corals: deep dive

RTN or STN — and is it even that?

Before treating, it’s important to identify what you’re dealing with. A wrong diagnosis leads to the wrong treatment.

RTN (Rapid Tissue Necrosis) progresses in hours. The tissue can literally detach in large sheets and “fly off” in the current, and an entire coral can lose its tissue within a single day. This is an emergency where even minutes matter.

STN (Slow Tissue Necrosis) progresses over days or weeks. The loss typically advances from the base upward or from the branch base toward the tip. The slower pace gives more time to find the cause and react, but the outcome is the same if it isn’t stopped.

In both, the hallmark is a clear boundary line between living tissue and bare white skeleton. This line is the most important thing to monitor: when it stops and stays put, the loss is under control; when it advances, the situation is still active. Practical tip: photograph the coral from the same angle daily, so you can track the progress reliably rather than by eye.

Before you reach for treatment, rule out other causes of tissue loss that require different action:

• Bleaching: the coral loses its colour but the tissue stays on the skeleton. RTN/STN exposes the skeleton entirely. The cause and treatment are different.
• Pest damage: AEFW (Acropora-eating flatworms), red bugs and other parasites leave pinpoint or patchy damage, and tissue may be missing irregularly. Examine the coral and its base under magnification, or dip a suspect specimen and see whether pests come off.
• Allelopathy burn: a neighbouring coral’s sweeper tentacle or chemical warfare causes localised damage right at the contact point, not an advancing boundary line across the whole coral.
• “Burnt tips” from an alkalinity spike: too rapid a rise in alkalinity can scorch SPS tips — this can trigger STN, but the tip damage itself is a separate issue.

More on this: LPS corals in practice

Why tissue is lost — root causes

The root cause is almost always a condition that weakens the coral. Work through these systematically, because identifying this is what decides whether the disease returns after treatment.

Parameter swing. A rapid change in alkalinity disturbs the coral’s calcium budget and is one of the most common triggers — especially a sudden rise. Common causes are a dosing-pump failure, an incorrectly mixed addition or too large a single dose. Keep the daily change under 0.5 dKH; stability is everything here.

High temperature. Pathogens, Vibrio in particular, accelerate rapidly above 25 °C. Summer heat, excessive heat from the light fixture or a cooling failure are typical triggers. Keep the temperature stable and preferably at most 25–26 °C.

Low pH. Many pathogenic bacteria, including Vibrio, thrive in low pH. This is not about pH’s effect on the coral itself but about low pH creating a favourable environment for the pathogen. The morning pH (the daily low) should not drop below 7.8.

Trace-element imbalance. Both deficiency and excess have been linked to tissue loss. Excess barium (over 140–150 µg/l) and fluoride (over 2.5 mg/l) are toxicity limits, while a deficiency in several trace elements weakens the coral’s own defence — Reef Moonshiner’s highlights very low barium and bromine in particular, and the Fauna Marin ICP database gives a more detailed deficiency list (see below). These can’t be seen with ordinary tests — a reliable ICP test that also reports fluoride is needed.

Amino-acid and carbon supplements. Some amino-acid products directly feed pathogenic bacteria, even when dosed to help the coral. The same applies to liquid carbon sources (vodka, vinegar, sugars) and biopellets: each feeds different bacterial species, and if a pathogen gets a head start, the biology turns the wrong way. This particularly explains sudden RTN crashes in nutrient-poor ULNS tanks, where beneficial bacteria can’t find enough nitrogen and phosphorus to keep pace with the pathogen.

Zero nutrients. Too-scarce nitrate and phosphate weaken both the coral and its beneficial microbiome. The coral needs phosphorus as a building block — a low phosphate reading alone is not enough. Don’t let nutrients stay unmeasurable for long.

Pests and local detritus. Parasites weaken the coral and open a route for bacteria into the tissue. Detritus accumulating in the colony causes localised infections — keep the flow such that debris doesn’t collect in the branch crotches.

Trace-element deficiency thresholds — the Fauna Marin ICP database

Fauna Marin’s ICP database, based on over 100,000 test samples, has identified which trace-element deficiencies are typically present when RTN or STN appears. A single deficiency is not enough to be the cause on its own, but the list works as a concrete checklist against your own ICP result.

RTN is typically associated with one or more of the following dropping below the threshold:

STN is typically associated with:

Low nickel recurs in both. If your ICP result shows several of these below the lower limit, correcting trace elements is part of the treatment — rather than relying on the Witchhazel treatment alone.

First aid — the first hours

Tissue loss progresses faster the longer you wait. Act immediately when you notice an advancing boundary line, and tailor your action to which type it is.

1. Isolate. Move the affected coral to quarantine or a separate container if possible. The disease can spread through the water, and isolating protects other corals and makes treatment easier.

2. Decide whether to cut — and how aggressively. This is the key decision:

• In RTN, act aggressively. Cut the healthy tissue clearly above the boundary line — better too much than too little, since the infection has often advanced further in the tissue than the eye can see. The goal is not to save the whole coral but to save the clean portion.
• In STN, you often have time to treat the coral whole and monitor the boundary line. If it still advances despite treatment, cut.

The cut frag can be glued to a new base only once the boundary line has stopped. Dipping the whole coral is often more effective than dipping frags, because the infection spreads inside the coral.

3. Dip. Use a gentle dip (see the Witchhazel dip below). Don’t put frags from different corals in the same dip container, and discard the dip water after use.

4. Check and stabilise conditions — carefully. Measure alkalinity, temperature and pH immediately. If something has swung, return it slowly to normal. Don’t make a sudden correction: a rapid opposite change stresses the already weakened coral further. Also avoid large water changes mid-crisis — they can shock a sensitive system.

5. Order an ICP test. Start investigating the root cause right away, and don’t wait for results before beginning treatment.

The Witchhazel tank treatment

The best-documented treatment is the Witchhazel tank treatment developed by Reef Moonshiner’s (Andre). It is a tannin-based treatment dosed into the whole tank that slows bacterial activity without harming fish, invertebrates or coral tissue — buying time for the tank’s own beneficial microbiome to recover and the coral to restore its defence. The success rate is reported to be high, but on one condition: Witchhazel alone, without correcting the other factors, often fails. The method’s author stresses that every step below is part of the whole — when the treatment has failed, the reason has almost always been skipping one of the steps.

The product — and what a European uses

The method is tied to one product: T.N. Dickinson’s Witch Hazel, “blue label”. The author explicitly warns not to deviate from the brand, because no other products have toxicity reports of working. It is a US, USP-grade witch hazel distillate, preserved with about 14 % alcohol. In Europe it’s hardly sold in stores, so it’s ordered online. An ordinary pharmacy witch-hazel water should not be assumed equivalent — its alcohol content and processing vary, and it hasn’t been tested with the same success.

Tank dosing

• Dose: about 5 ml / 100 l of total water volume, every evening after lights-out.
• Dose only in the dark. The distillate contains alcohol, and dosed under light it would cause a bacterial bloom like a vodka dose.
• Duration: at least 5–7 days, extended to two weeks if no improvement is seen.
• First day: the dose can be doubled if the loss is progressing rapidly. This is optional but well documented.
• Dose straight into the display tank. The skimmer, pumps and heater keep running normally.

Dosing warning: one documented failure was caused by confusing gallons and litres in the calculation, leading to a fourfold overdose, a bacterial bloom and cloudy water. Check the unit carefully — the dose is in litres, not gallons.

What to suspend during treatment

Tannins are the active part of the treatment, so anything that removes them must be kept off:

• No activated carbon, ozone or UV — they remove the tannins and nullify the treatment. This is a deliberate exception to normal filtration; see activated carbon in practice.

In addition, suspend everything that can feed pathogens:

• No amino-acid products — no exceptions.
• No liquid carbon sources (vodka, vinegar, sugars). Remove biopellets from the reactor and dry them — don’t leave wet pellets in the reactor.
• No enzyme or bacteria-feeding products, and no vitamins during the treatment.

Support measures during treatment

• Keep the morning pH at least 7.8 and maximise the skimmer’s air intake. Clean the skimmer nozzles daily — salt creep can reduce airflow by 20–40 % within a day.
• Keep the temperature at most 25 °C throughout the treatment.
• Keep alkalinity stable and measure it once a day at the same time.
• Manage nutrients. Don’t let nitrate and phosphate hit zero, but also don’t raise them quickly (at most 0.5 ppm nitrate and 5–7 ppb phosphorus per day).
• Dose a clear probiotic bacterium (e.g. Biodigest): a 3–4× dose on the first day of treatment, then a maintenance dose daily or every other day. Dose at a different time than the Witchhazel, in the morning or afternoon. Avoid cloudy-looking bacterial products containing enzymes — they have been reported to cause harm during the treatment.

The Witchhazel dip for individual frags

The dip can be used on cut frags alongside the tank treatment, but the tank treatment must still be done — dipping alone often leads to reinfection, because the disease is already in the tank.

• Mix: 5–10 ml of Witchhazel / 1 l of tank water in a clean container.
• Bathe the frag in the solution for 30 minutes, swirling occasionally. Tissue and jelly may come off — this is already-lost tissue that the dip removes mechanically.
• Then rinse the frag in a second container of clean tank water. Discard both the dip and the rinse water after each use; don’t reuse.
• Don’t mix frags from different corals in the same dip.

What to expect

Don’t expect the loss to stop immediately. Recovery can take 1–5 days depending on the bacterial strain and the depth of the infection — sometimes the boundary line stops the next day, sometimes only days later. Many corals open up better as soon as the morning after the first dose. During the day the pH drops slightly and alkalinity consumption decreases as bacterial activity slows. A clear, stationary boundary between remaining tissue and bare skeleton is a reliable sign that the loss has stopped. Not all frags survive: if the infection has penetrated deeply, some tissue is lost despite treatment.

What to avoid

• Antibiotics in a closed tank. Antibiotic treatment is not effective long-term in a closed reef aquarium: the system can’t purge antibiotic-resistant organisms, and treatment often causes more problems than it solves. Historically used cipro and amoxicillin dips can halt the symptom briefly but don’t fix the root cause.
• Hydroxide-based pH raising (kalkwasser, sodium or potassium hydroxide) mid-crisis. The strong reaction on the carbonate balance stresses an already weakened coral too much. Find the cause of the low pH (poor gas exchange, excess carbon dioxide) instead of masking it.
• Large water changes mid-crisis. A sensitive, stressed system does not benefit from a shocking change.
• Dipping alone or Witchhazel alone without correcting the root cause. This is consistently the reason when treatment fails.

Prevention

The most effective treatment is to prevent the situation before it arises. Tissue loss is almost always a consequence of something that could have been prevented.

• Stability above all. Keep alkalinity, temperature and salinity steady. Reliable dosing, automation and dosing-pump redundancy help avoid exactly the swings that most often trigger the loss.
• Don’t strip the tank of nutrients. Keep nitrate and phosphate at a measurable, stable level. Be wary of carbon dosing in a nutrient-poor tank.
• Track trace elements. Regular ICP testing reveals imbalances before they escalate into tissue loss.
• Quarantine and dip. Every new coral is dipped against pests and disease before going into the display tank.
• Observe corals daily. Tissue retraction, fading colour or closing polyps are early signs. The sooner you address the cause, the more likely you are to avoid the loss entirely.

More on this: LPS corals in practice · Coral dipping and quarantine protocol

References

Peer-reviewed studies

• Sweet, M., Craggs, J., Robson, J., & Bythell, J. (2013). Assessment of the microbial communities associated with white syndrome and brown jelly syndrome in aquarium corals. Journal of Zoo and Aquarium Research, 1(2), 67–74.
• Luna, G. M., Bongiorni, L., Gili, C., Biavasco, F., & Danovaro, R. (2010). Vibrio harveyi as a causative agent of the White Syndrome in tropical stony corals. Environmental Microbiology Reports, 2(1), 120–127.

Hobbyist literature and sources

• Reef Moonshiner’s (Andre). Witchhazel Reeftank Treatment — RTN/STN Resolution. https://reefmoonshiners.com/pages/witchhazel-reeftank-treatment
• Fauna Marin. ICP-OES laboratory database: RTN and STN prediction (analysis of trace-element deficiencies based on over 100,000 test samples). https://www.faunamarin.de/
• Aslett, C. G. (2024). Coral Immunity, Part I–III. Reef Ranch Publishing. https://www.reefranch.co.uk/
• Reef Builders (2023). Managing Tissue Necrosis. https://reefbuilders.com/2023/04/10/managing-tissue-necrosis/

Books

• Borneman, E. H. (2001). Aquarium Corals: Selection, Husbandry, and Natural History. T.F.H. Publications.
• Borneman, E. H., & Lowrie, J. (2001). Advances in captive husbandry: easily utilized reef replenishment. Aquarium Sciences and Conservation.

Hobbyist communities

• Humble.Fish & Reef Community. RTN/STN treatment threads. https://humble.fish/
• Reef2Reef Community. RTN and STN discussions. https://www.reef2reef.com/