Why Is My Coral Bleaching? Causes, Recovery, and
Quick Summary
Coral bleaching is when a coral expels its symbiotic zooxanthellae and turns pale or white. It is not a disease. It is a stress response. The coral is still alive, but it has lost the organisms that provide most of its energy.
If you are seeing bleaching in your reef tank, here is what to know immediately:
- Bleaching is reversible if the stress source is identified and removed quickly enough.
- The coral is not dead yet. White tissue still has living polyps. Only bare skeleton means it is gone.
- Do not panic-dose or make dramatic changes. Rapid corrections often cause more harm than the original stress.
- Identify the trigger first. Treating the wrong cause wastes time the coral does not have.
What Most Reefers Get Wrong
Most aquarists see bleaching and immediately assume something is too high or too low. They test parameters, find one slightly off, and start correcting aggressively. This instinct is understandable, but it is often the wrong move.
Bleaching is almost never caused by a single parameter sitting at the wrong number. It is caused by change. A swing in alkalinity from 9 to 7 dKH over two days will bleach coral that was perfectly happy at either number held steady. A lighting upgrade that doubles PAR overnight will bleach coral that would have adapted to the same intensity over weeks.
This is the pattern that separates experienced reefers from those still learning: bleaching is a stability problem, not a parameter problem. If you only look at where your numbers are right now, you will miss what actually happened.
What Is Actually Happening When Coral Bleaches
When you look at a healthy coral, its colour comes from zooxanthellae, microscopic dinoflagellates living inside the coral tissue. These organisms photosynthesize, converting light into energy and passing most of it to the coral host. In return, the coral provides shelter, CO2, and nutrients. This relationship is what makes reef ecosystems possible.
When the coral becomes stressed, this partnership breaks down. The zooxanthellae either malfunction (producing toxic oxygen radicals instead of useful energy) or the coral actively expels them as a survival mechanism. Without zooxanthellae, the coral's transparent tissue reveals the white aragonite skeleton underneath. That is bleaching.
This is why bleached coral can still be alive. The animal itself, the polyps, the tissue, the mouth, are all still there. But it has lost its primary energy source. A bleached coral is starving. It can survive for weeks on heterotrophic feeding (catching food particles), but if the zooxanthellae do not recolonize the tissue, the coral will eventually die.
In practice, you have a window. How long depends on the species, how severe the bleaching is, and how quickly you address the cause.
The Real Causes of Coral Bleaching in Reef Tanks
Bleaching triggers in home aquariums are different from what causes bleaching on natural reefs. In the ocean, mass bleaching events are driven by sustained elevated temperatures across entire regions. In your tank, the causes are more varied and almost always tied to something that changed recently.
Temperature Stress
Temperature is the most well-known bleaching trigger, and for good reason. Corals in reef tanks tolerate 76 to 78 degrees Fahrenheit. Even a rise to 82 degrees sustained over 48 hours can initiate bleaching in sensitive species.
In most home tanks, temperature stress comes from:
- Heater malfunction (stuck in the on position)
- Ambient room temperature spikes during summer
- Equipment heat buildup in enclosed cabinets
- Power outages followed by rapid reheating
You will often notice that SPS corals bleach first during temperature events. Soft corals and many LPS are more tolerant, which is why a bleaching event may affect some colonies and not others in the same tank.
Light Shock
Lighting changes are the second most common cause of bleaching in home reef tanks, and the one most often overlooked. When you upgrade lights, increase intensity, reduce the photoperiod ramp time, or even clean a heavily salt-crusted lens, the corals receive significantly more PAR than they are adapted to.
Zooxanthellae acclimate to specific light levels. When intensity jumps suddenly, they produce excess oxygen radicals that damage the coral tissue. The coral expels them in self-defence.
This explains why corals bleach after light upgrades even when the new PAR values are technically "correct" for the species. The issue is not the final intensity. It is the rate of change.
Alkalinity Swings
Alkalinity instability is the silent killer in reef tanks. A slow drift from 8.5 to 7.0 dKH over a week will stress every stony coral in the system. A rapid correction back up to 9.0 in a single day can push already-stressed corals into bleaching.
In most cases, alkalinity swings happen because of:
- Irregular dosing schedules
- Running out of dosing solution and resuming days later
- Switching salt brands with different alkalinity levels
- Large water changes with improperly mixed saltwater
This is what causes the frustrating pattern where corals bleach right after a water change. The aquarist thinks they are helping the tank. In reality, the new water had significantly different alkalinity than the tank water, and the swing triggered the stress response.
Chemical Contamination
Corals are extraordinarily sensitive to chemicals that would not bother fish. Copper, heavy metals, airborne contaminants (aerosol sprays, cleaning products, fresh paint fumes), and even hand lotions can trigger bleaching.
If bleaching appears suddenly across multiple species without any parameter changes, contamination should be high on the suspect list. Running activated carbon aggressively and performing water changes with clean RO/DI water is the standard response.
Coral Warfare
When corals are placed too close together, they engage in chemical warfare. Some species release terpenes and other allelopathic compounds that damage neighbouring coral tissue. Leather corals are notorious for this, and a large leather coral in a small tank can produce enough chemical output to stress SPS colonies on the opposite side of the aquarium.
Almost always, this type of bleaching is localised. The affected coral is the one closest to or downstream from the aggressor. If only one colony is bleaching and it is positioned near a leather, toadstool, or pulsing xenia, proximity warfare is the likely cause.
How to Diagnose What Triggered Bleaching
When bleaching appears, resist the urge to start changing things immediately. Instead, work through this diagnostic process.
The first question to ask is not "what are my parameters right now?" It is "what changed in the last 7 to 14 days?" Bleaching is a delayed response. The trigger often happened days before the visual signs appeared.
Run through this checklist:
- Did the temperature spike or drop? Check your heater, chiller, and any records. If you use a controller with logging, review the temperature graph.
- Did lighting change? New fixture, new settings, cleaned lens, repositioned light, or even a bulb replacement on T5 systems.
- Did alkalinity swing? Review your dosing log. Did you miss doses? Switch salt brands? Do a large water change?
- Did you add anything new? New coral, new fish, new equipment, new supplement, new salt mix.
- Were there airborne contaminants? Painting nearby, aerosol use, insect spray, candles, air fresheners.
- Is it localised or tank-wide? One colony suggests placement, warfare, or flow issues. Multiple colonies suggest a systemic trigger (temperature, alkalinity, contamination).
This is usually where the answer becomes clear. In practice, the cause is almost always something that changed, not something that was wrong all along.
How to Give Bleached Coral the Best Chance of Recovery
Once you have identified and removed the stress source, the recovery phase begins. This is where patience matters more than action.
A bleached coral is energy-depleted. It cannot photosynthesize because it has expelled its zooxanthellae. It can feed heterotrophically, but this provides far less energy than the symbiotic relationship. Your job is to create conditions that allow zooxanthellae to slowly recolonize the tissue.
Here is what actually helps:
- Reduce light intensity by 30 to 50 percent. Bleached tissue has lost its UV protection (zooxanthellae absorb excess light). High light on bleached tissue causes further photodamage. Gradually increase light over weeks as colour returns.
- Stabilize parameters. No corrections, no changes, no experiments. Keep alkalinity, calcium, and temperature as steady as possible. Stability is the priority.
- Target feed the coral. Small amounts of reef-specific food (amino acids, microplankton, or coral food) delivered directly to the polyps with a pipette. Do not overfeed the tank to compensate.
- Maintain water quality. Keep phosphate and nitrate in range. Run activated carbon. Continue your normal water change schedule.
- Do not move the coral. Changing its position adds another stress factor. Leave it where it is unless its current location was the cause (too close to another coral, directly under new lighting).
Recovery takes weeks to months. Colour returns gradually as zooxanthellae repopulate the tissue. You may see brown colouration first (a common zooxanthellae-dense recovery phase) before the coral regains its original pigment. This is normal and actually a good sign.
If the coral's tissue begins to recede from the skeleton (exposing bare white skeleton rather than pale but intact tissue), the prognosis is poor. At that point, the coral is dying, not just bleached.
When Bleaching Is Not Actually Bleaching
Not every colour loss is bleaching. In most reef tanks, there are other reasons a coral may turn pale or change colour, and misidentifying them leads to the wrong response.
Acclimation colour shift. New corals often change colour in the first few weeks as they adjust to your lighting. A coral that was brown under a dealer's low light may pale slightly under stronger light, then develop its true colours over time. This is normal adaptation, not stress.
Overfeeding nutrient drop. Corals kept in high-nutrient systems (elevated nitrate and phosphate) often appear deeply coloured because their zooxanthellae are dense and overfed. Moving them to a cleaner system causes a colour shift as zooxanthellae density adjusts. This is not bleaching.
Seasonal ambient light changes. Tanks near windows may receive different levels of ambient light across seasons. A coral that was fine all winter may show signs of light stress in summer when the room receives more natural daylight.
Advanced: The Biology of Zooxanthellae Expulsion
At the cellular level, bleaching is a breakdown in the oxidative balance between coral and zooxanthellae. When zooxanthellae are stressed (by heat, light, or chemical triggers), their photosynthetic machinery malfunctions. Instead of producing useful sugars, they generate reactive oxygen species (ROS), molecules that damage cell membranes and proteins.
The coral host detects this oxidative damage and responds by expelling the zooxanthellae through exocytosis, digestion, or detachment of the cells containing them. This is an active response, not passive. The coral is removing a partner that has become toxic.
This is why the trigger does not need to be extreme. Even moderate stress, if sustained, can tip the photosynthetic balance far enough to initiate expulsion. It also explains why recovery is slow: the coral must re-establish trust (biochemically speaking) with new zooxanthellae that are better adapted to current conditions.
Different clades of zooxanthellae (Symbiodiniaceae family) have different temperature and light tolerances. During recovery, the coral may acquire a different clade than it originally hosted, which sometimes results in permanent colour changes even after full recovery.
Advanced: Pattern Recognition in Recurring Bleaching
If bleaching keeps happening in your tank, the problem is systemic, not isolated. Recurring bleaching points to one of these patterns:
- Dosing instability. Your alkalinity is not as stable as you think. It may test fine at the same time each day but swing between doses. Testing at multiple times reveals the pattern.
- Temperature creep. Your heater or chiller cycles create micro-swings that accumulate. A controller with logging will show whether temperature holds steady or oscillates.
- Light degradation or inconsistency. LED drivers age and output changes over time. A PAR meter check every 6 months catches drift before corals react.
- Chronic low-level contamination. Copper from plumbing, silicone off-gassing from recent repairs, or slow-leaching materials in the sump can maintain a low-grade stress that manifests as periodic bleaching in the most sensitive species.
In practice, the reefers who solve recurring bleaching are the ones who start logging. Daily parameter readings, equipment checks, and a habit of noting every change (no matter how small) build the dataset you need to find patterns.
Common Myths
"Bleached coral is dead." Bleached coral is stressed and starving, but the tissue is alive. If the white areas are smooth tissue (not exposed rough skeleton), the coral can recover. Only bare skeleton with no tissue means the coral is dead in that area.
"Bleaching means your parameters are bad." Bleaching is caused by changes in parameters, not by specific values. A tank sitting steadily at 7.5 dKH will keep corals healthier than one swinging between 8 and 10. Stability matters more than perfection.
"Turn the lights off completely to help bleached coral." Complete darkness for extended periods stresses coral further. The remaining zooxanthellae still need some light to function. Reduce intensity, do not eliminate it entirely.
"You should dip bleached coral." Coral dips are for pest removal, not stress recovery. Dipping a bleached coral adds chemical stress to an already compromised organism. Unless you see clear signs of pests, leave the coral alone.
"More feeding will save a bleached coral." Target feeding helps, but overfeeding fouls the water. Elevated nutrients from excess food can compound the problem. Feed small amounts directly to the polyps, and maintain your normal water change schedule.
FAQ
How long does it take for bleached coral to recover? Recovery typically takes 4 to 12 weeks, depending on species, severity, and how quickly the stress source was removed. Colour returns gradually. SPS corals generally take longer than soft corals.
Can a fully white coral survive? Yes, if the tissue is still intact. A fully bleached coral with smooth white tissue is alive and can recover. Run your finger gently near it (without touching): if polyps retract, the coral is alive. If you see bare, rough skeleton with no tissue layer, that section is dead.
Should I frag a partially bleached coral? Only if part of the colony is healthy and the bleached portion is clearly dying (tissue recession, not just pale). Fragging adds stress, so only do it as a last resort to save the healthy section.
Will bleached coral infect other corals? No. Bleaching is not contagious. It is an individual stress response. However, if the trigger is systemic (temperature spike, alkalinity crash), multiple corals may bleach from the same cause.
How do I prevent bleaching? Stability is prevention. Keep alkalinity within 0.5 dKH of your target daily. Use a controller or monitor for temperature. Acclimate new corals and lighting changes slowly. Run activated carbon. Test regularly and log everything.
My coral turned brown after bleaching. Is that bad? No. Brown colouration during recovery is normal. It means zooxanthellae are repopulating the tissue densely. The coral will often regain its original colour over the following weeks as the zooxanthellae density balances out. Brown is better than white.
Can I add zooxanthellae to help recovery? You cannot dose zooxanthellae directly. The coral must re-acquire them naturally from the water column or from residual populations within its tissue. Your job is to provide stable conditions that allow recolonization to happen.