Coral Bleaching vs STN vs RTN: How to Tell Which One You Have

You turn the lights on and a branch that was brown yesterday is white today. The question you have about four hours to answer correctly is whether that coral is bleached, which is survivable, or whether it is shedding tissue, which usually is not. They look similar in a photograph. They are not remotely the same problem, and the correct response to one is the wrong response to the other.

What is the difference between bleaching, STN and RTN?

Bleaching is the loss of the coral’s symbiotic algae. The tissue is still there, still alive, and has simply gone transparent so the white skeleton shows through. STN and RTN are tissue death: the tissue detaches from the skeleton and comes off, leaving bare bone. STN does it over days to weeks, RTN over hours to a couple of days. Bleached corals frequently recover. Corals shedding tissue usually do not, unless you cut ahead of it fast. Everything else in this guide follows from that single distinction, so it is worth being blunt about it: is the tissue on the skeleton, or off it?

	Bleaching	STN	RTN
What is happening	Zooxanthellae expelled or lost. Tissue intact and alive.	Tissue dying and detaching along a slow front.	Tissue sloughing off the skeleton wholesale.
Tissue on skeleton?	Yes — transparent, not absent	No, behind the front	No
Speed	Days; can be sudden after a shock	Weeks to months	Hours to about two days
Polyp extension	Often still present	Absent behind the front	Absent
Loose tissue in the water	No	Rarely, small amounts	Yes — visible sheets and strands
Spreads to neighbors	Only if the cause is shared	Uncommon	Often, within 24–48 hours
Typical outcome	Recovery over weeks to months	Salvageable if caught	Colony usually lost; frags may survive

How do I tell if my coral is bleached or losing tissue?

Three checks, in order, and none of them need equipment. Look for the front. Necrosis has an edge — a defined line between pigmented tissue and bare white, and it moves. Bleaching does not have an edge; it fades. If you can point at a boundary and it is in a different place tomorrow, that is tissue loss. Look for polyps. A bleached coral has lost its algae, not its polyps, and will often still extend and still eat. Bare skeleton has no polyps to extend. Careful with this one though — stressed corals can stay tightly contracted for long periods, so absent polyps alone does not prove the tissue is gone. Wait a day and look for film. This is the best trick in the hobby and it comes from Eric Borneman: diatoms and algae will not settle on living coral tissue, but they colonize exposed skeleton readily and are visible within about a day. Clean white after 48 hours means tissue is still there. A brown or green haze means the skeleton is bare.

How fast does RTN actually move?

Fast enough that finding it in the morning often means finding it too late. The commonly repeated figure is that RTN can strip a colony in under 24 hours, with 24 to 48 hours as the usual window. Worth being honest about where that number comes from: it is hobby convention, repeated across retailer and dip-manufacturer pages, not a figure from any published study. It matches what people report, but treat it as a rule of thumb rather than a measurement. STN is the opposite problem — it is slow enough that people talk themselves out of acting. The most useful working threshold we have seen is roughly a millimeter of recession per day or less. That is one experienced reefer’s rule rather than a consensus standard, but it gives you something to measure against instead of squinting at the colony each evening wondering whether it moved. One detail that surprises people: frags die faster than colonies. Less tissue mass, less reserve, less distance for the front to travel.

What causes each one?

	Usual triggers
Bleaching	Temperature swing in either direction; light shock, especially a coral moved up the rock or a new fixture; alkalinity swing; nutrients driven too low for the zooxanthellae to sustain themselves
STN	Chronic low-grade stress rather than one event: unstable alkalinity over days, sustained ultra-low nutrients, irritation at the base from neighbors or detritus, dead flow at the base
RTN	Acute insult: large alkalinity or salinity swing, temperature spike, stray voltage, phosphate crashed too fast by a GFO overdose, flatworm damage, shipping stress, bacterial infection

Note that cold bleaches too. NOAA documented a Florida Keys bleaching event driven by water around 6.7 °C below normal January temperatures. Down is as dangerous as up.

Is RTN a bacterial infection?

Partly, and the honest answer is that this is still argued. Bacteria of the genus Vibrio are consistently associated with tissue necrosis in stony corals, and the most useful piece of science here explains something the hobby usually gets wrong — that bleaching and necrosis can be the same organism at different temperatures. Kimes and colleagues, publishing in The ISME Journal in 2012, found that Vibrio coralliilyticus is essentially harmless at or below 24 °C. Between roughly 24 and 26.5 °C it attacks the zooxanthellae, which presents as bleaching. Above 27 °C it switches on 136 virulence genes and begins lysing coral tissue outright, which presents as necrosis.

What that chart should change about your husbandry is simple: a heater failure that takes you from 80 °F to 84 °F is not just “a bit warm.” It moves you across a threshold where an organism already in your tank changes behavior. Where the argument continues is whether these bacteria are the cause or the consequence. Diseased corals carry more bacteria, but no single organism turns up reliably in the wounded tissue, which points at a disrupted microbial community rather than one culprit. Sweet and Bythell’s 2015 work on white syndrome in Acropora muricata lands in the same place: non-specific bacterial infection, followed by ciliates eating the damaged tissue. The ciliates you may see under magnification are almost certainly arriving after the damage, not causing it.

What parameters should I actually be holding?

Stability matters more than the specific number in every row below. A tank held steadily at the low end of a range beats a tank that swings across the middle of it.

Parameter	Working range	The part that actually causes trouble
Temperature	76–82 °F	Daily swing. Around ±1 °F is unremarkable; ±3–4 °F is a stressor. Sustained above 82 °F is where necrosis risk climbs sharply.
Alkalinity	7–11 dKH; 7–8 in low-nutrient SPS systems	Rate of change. Keep it under about 0.5 dKH per day, tighter if you keep Acropora. This is aggregated hobby practice rather than a published limit, but it is the most consistently repeated number in the hobby for a reason.
Salinity	34–36 ppt (1.025–1.027)	Match new water within about 0.002 SG. Top-off failures do more damage than dosing errors.
Nitrate	5–50 ppm	Zero is not clean, it is starvation. Without dissolved nitrogen the zooxanthellae cannot repopulate, so a bleached coral in a zero-nitrate tank stays bleached.
Phosphate	0.06–0.3 ppm	Crashing it fast with GFO is a recognized RTN trigger. Bring it down slowly or not at all.
PAR, Acropora	200–300, some to 450	Increase over weeks, never days. A coral moved up the rock is the most common self-inflicted bleaching we see.
PAR, LPS	Euphyllia 100–250; Trachyphyllia 50–150	Same rule. Torches and hammers bleach from being promoted too quickly.

Worth knowing that the tight-temperature school and the stable-but-wider school genuinely disagree here. Randy Holmes-Farley runs 80–81 °F year round and argues that corals acclimated to mild daily swings handle unexpected excursions better. Others hold 75–77 °F and ±1 °F. Both camps grow beautiful corals. What neither camp does is let the number wander.

What do I do right now?

If it is bleached

Find the stressor and fix that one thing. Do not correct temperature, light and chemistry simultaneously; you will add instability to an animal that is already short of reserves. Then reduce the light. This is counterintuitive and it is the single most common mistake. A bleached coral has no symbionts left to use the light, so extra intensity is pure photodamage on top of the original injury. Move it lower or shade it. Feed the tank. Dissolved nitrogen is the fuel for zooxanthellae to repopulate. A bleached coral in a zero-nutrient system has nothing to rebuild with. This is the moment to stop chasing ultra-low numbers. Do not frag it. Do not dip it. Both add stress and neither addresses symbiont loss.

If it is STN

You have days, not hours, so use them to find the cause rather than reaching for treatments. Log alkalinity over several days — a single reading tells you nothing about stability. Check the base for warfare from a neighbor, for detritus, for dead flow. Gel superglue applied directly onto the receding edge halts base recession more often than it has any right to, and it buys you time. The caveat that goes with it is absolute: recession happens for a reason, and glue over an unfixed cause just moves the problem a centimeter.

If it is RTN

Move fast and accept that you are trying to save a piece, not the colony.

1. Get the colony out of the display. RTN spreads to neighbors, frequently within a day or two. Removal comes before diagnosis.
2. Cut ahead of the front, into clean tissue. Half an inch is the figure most commonly given; half a centimeter is the bare minimum anyone recommends. Give yourself the extra distance if the branch can spare it.
3. Blow the loose tissue off before it becomes a bacterial substrate, and rinse the frag in clean saltwater.
4. Run fresh carbon and turn the skimmer up. The dissolved organic load from a colony shedding tissue is substantial.
5. Site the frags well away from wherever the mother colony was.

Does dipping help?

For RTN, mostly no, and we would rather say that plainly than sell you an expectation. Coral dips are built to kill pests — flatworms, nudibranchs, red bugs — and they do that well. They are not designed for a bacterial process already underway inside the tissue. Plenty of experienced reefers report peroxide, iodine and commercial dips all failing on an active RTN colony. Antibiotic dips have the strongest anecdotal record, and we are deliberately not publishing doses. Antibiotic use drives resistance, will destroy your biological filtration if it reaches the display, and in most places is not something a hobbyist can legally source for this purpose. If you are going down that road it is a conversation with a vet, not a blog post. Where dipping genuinely earns its place is before any of this: on every new coral, every time. Most of what triggers tissue loss walks in on a frag plug.

Which corals are most at risk?

Group	Risk	Notes
Acropora	Highest	First to go, every time. Thin-branched and tabling species worst. Wait until a tank is around eight months old before adding them.
Montipora, Stylophora, Pocillopora, birdsnest	Moderate	Tolerate 1–1.5 dKH swings and 100–150 PAR that would ruin an Acropora. Usually show slow base recession rather than sudden collapse.
LPS — Euphyllia, acans, favia	Lower	Bleach readily but recover well. Their equivalent emergency is brown jelly, which is a different process with a different response.
Softies, zoanthids, mushrooms	Lowest	They close, melt or shed, but the RTN vocabulary does not really apply.

Will it come back?

A bleached coral with intact tissue, in a tank where the stressor has been removed and there is nitrogen available, has a genuinely good chance. Recoloring takes weeks to months and there is no reliable schedule — recovery depends on how severe the bleaching was and what conditions follow it. We have had frags come back with better color than they went in with. STN caught early is often salvageable. RTN usually is not, and the reason is worth understanding: by the time tissue is visibly sloughing, a good deal of tissue further down the branch is already dead and simply has not detached yet. That is why frags cut too close to the front keep dying. You are not fighting the front you can see, you are fighting the one you cannot.

The Underwater Gardener grows aquacultured corals in Key West, Florida (FL Aquaculture Certificate #AQ0254072). Our corals are raised under stable reef lighting in our own systems, so they arrive already adapted to a well-run tank. Corals ship UPS Next Day Air every Tuesday and Wednesday.

Sources: Kimes et al., The ISME Journal 6(4), 2012, on temperature-regulated Vibrio coralliilyticus virulence; Sweet & Bythell, Molecular Ecology, 2015, on non-specific bacterial infection and ciliate histophagy in white syndrome; Eric Borneman on bleaching versus tissue loss; Randy Holmes-Farley on optimal reef parameters and nutrient targets; NOAA Ocean Service on coral bleaching.