Coral Bleaching Causes: Why Corals Turn White and
Quick Summary
Bleaching is not a disease. It is a stress response. When a coral expels its symbiotic zooxanthellae (the photosynthetic algae that live inside its tissue and provide most of its energy and color), the tissue becomes transparent and the white calcium carbonate skeleton shows through. The coral is not dead, but it is in serious trouble. Without its zooxanthellae, a coral loses its primary energy source and will starve if the stress is not removed and the algae do not recolonize.
Most bleaching in reef aquariums comes from temperature spikes, sudden light changes, or water chemistry instability. The trigger is almost always identifiable, and in most cases, recovery is possible if you act quickly to stabilize conditions.
What Actually Happens During Bleaching
Most reef keepers recognize bleached coral when they see it, but few understand the biology behind it. What looks like the coral losing color is actually the coral ejecting living organisms from its own tissue. That distinction matters because it changes how you respond.
Corals host millions of single-celled algae called zooxanthellae (genus Symbiodinium and related genera) within their tissue cells. These algae photosynthesize using the coral's light exposure and transfer up to 90 percent of the sugars they produce directly to the coral host. In return, the coral provides shelter, CO2, and nutrients. This symbiosis is the foundation of coral biology.
When the coral experiences stress that damages the zooxanthellae or the photosynthetic process itself, the relationship breaks down. Damaged zooxanthellae produce reactive oxygen species (free radicals) that harm the coral tissue. The coral's response is to expel the damaged algae before they cause more harm. This is bleaching: a survival mechanism, not a failure.
The expelled zooxanthellae take their pigments with them. Coral tissue itself is nearly transparent. Without the golden-brown or green zooxanthellae, you see straight through the tissue to the white skeleton underneath. The coral is alive but functionally starving, running on whatever energy reserves its tissue holds.
This is why bleaching severity matters. A coral that has expelled some zooxanthellae (partial bleaching, visible as paling or loss of color in patches) retains some photosynthetic capacity and can recover relatively quickly. A coral that has expelled nearly all zooxanthellae (full bleaching, uniformly white) has almost no energy income and a narrow window for recovery before tissue death begins.
Temperature Stress
Temperature is the most common trigger for coral bleaching in reef aquariums. Corals are adapted to a narrow thermal range, and excursions outside that range stress the photosynthetic machinery of the zooxanthellae.
Heat Stress
When water temperature rises above 82°F (28°C) for most reef species, zooxanthellae photosynthesis becomes less efficient and begins producing reactive oxygen. The higher the temperature and the longer the exposure, the more damage accumulates.
In a reef tank, heat spikes commonly occur from heater malfunctions (stuck in the on position), room temperature increases during summer, equipment failure (chiller breakdown, fan failure), or extended power outages where cooling systems stop but ambient heat persists.
A temperature spike to 84°F (29°C) for a few hours may cause mild paling in sensitive SPS. A spike to 86°F (30°C) for 12 or more hours can trigger severe bleaching across the tank. The speed of the temperature change matters almost as much as the peak. A gradual rise over days is less damaging than the same peak reached in hours.
Cold Stress
Less common in aquariums but equally damaging. Water temperatures below 72°F (22°C) slow coral metabolism and zooxanthellae function. Extended cold exposure (heater failure during winter, power outage in a cold room) triggers bleaching through the same oxidative stress pathway as heat, just from the opposite direction.
Prevention
Temperature stability is a non-negotiable requirement for coral health. The tools that prevent temperature-driven bleaching:
- A quality heater with a reliable thermostat (or a standalone temperature controller for redundancy)
- A chiller, fan-based cooling system, or room air conditioning for summer months
- An auto top-off system (evaporation provides some natural cooling but increases salinity)
- A temperature alarm or smart controller that alerts you to deviations
- A battery backup or generator for critical equipment during power outages
In practice, the reef keepers who never experience temperature-related bleaching are the ones who treat temperature control as infrastructure, not an afterthought. A $30 temperature controller with an alarm is cheaper than replacing a single bleached coral colony.
Light Shock
Light is the second most common bleaching trigger in home reef tanks, and it catches people off guard because the light itself looks the same. The damage comes from change, not from the light level itself.
Sudden Intensity Increase
When a coral adapted to moderate light is suddenly exposed to high intensity, the zooxanthellae receive more photons than they can process. The excess energy generates reactive oxygen that damages both the algae and the host tissue.
Common scenarios that cause light shock:
- Upgrading to a significantly more powerful light fixture without reducing intensity
- Moving a coral from a low position to a high position without gradual acclimation
- Replacing a light that was slowly declining in output (aging LEDs or T5 bulbs) with a new fixture at full power
- Removing a coral from a shaded position after rockwork is rearranged
- Reducing the water level significantly (more light penetration)
Prolonged Excessive Light
Even without a sudden change, consistently high PAR above a coral's tolerance range causes chronic photoinhibition. The zooxanthellae operate under constant stress, producing reactive oxygen continuously. Over weeks, this chronic stress leads to gradual bleaching rather than the acute bleaching of a sudden spike.
SPS corals tolerate the highest PAR levels (up to 450 for many Acropora species) but still bleach above their limits. LPS corals and soft corals bleach at much lower intensities because their zooxanthellae populations are adapted to moderate light. A PAR level that is perfect for SPS can bleach an LPS placed in the same position.
Prevention
- Acclimate all new corals to your lighting gradually (start low, move up over 2 to 4 weeks)
- When upgrading lights, start at 40 to 50 percent intensity and increase 10 percent every 3 to 5 days
- Use a PAR meter or documented PAR charts to match coral placement to appropriate light levels
- Follow the placement guide vertical zoning principle: SPS high, LPS mid, soft corals low
Water Chemistry Instability
Parameter swings do not always cause visible bleaching on their own, but they weaken the coral-zooxanthellae relationship and lower the threshold at which other stressors trigger bleaching.
Alkalinity Swings
Rapid alkalinity changes are the single most disruptive parameter swing for stony corals. A drop or spike of 2 or more dKH over 24 hours stresses the calcification process and destabilizes the zooxanthellae symbiosis. SPS corals are most sensitive, with tissue recession and bleaching possible from swings as small as 1.5 dKH in a day.
Common causes: irregular dosing schedules, overdosing two-part solution, calcium reactor malfunction, or large water changes with salt mix at significantly different alkalinity than tank water.
Salinity Swings
Sudden salinity changes (from missed auto top-off refills, accidental freshwater addition, or adding concentrated salt directly to the display tank) stress all corals. A shift of more than 0.002 specific gravity in 24 hours can trigger bleaching in sensitive species.
pH Extremes
pH below 7.8 or above 8.6 stresses zooxanthellae function. Rapid pH shifts are more damaging than the absolute level. CO2 buildup in sealed rooms (common in winter when windows stay closed) can depress pH overnight, and kalkwasser overdose can spike pH suddenly.
Nutrient Crashes
Corals adapted to a certain nutrient level (even if it is elevated) can bleach when nutrients drop suddenly. This counterintuitive bleaching happens when aggressive nutrient export (new GFO media, carbon dosing, or oversized skimmer) rapidly strips nitrate and phosphate from the water. The zooxanthellae, adapted to the previous nutrient availability, are suddenly starved and become dysfunctional.
In practice, this means that lowering nutrients should always be done gradually. If your tank runs at 15 ppm nitrate and you want to reach 5 ppm, reduce by 2 to 3 ppm per week through moderate intervention rather than crashing to zero overnight with aggressive media.
Chemical Contamination
Corals are extraordinarily sensitive to dissolved chemicals that most other aquarium organisms tolerate. Contamination bleaching is often the hardest to diagnose because the source is invisible.
Common Contaminants
Copper. Present in many fish medications, copper piping, and some heater elements. Even trace amounts (0.01 to 0.05 ppm) stress corals. Tanks that have been treated with copper medication retain residual copper in substrate and rockwork, making them unsuitable for corals without complete replacement of those materials.
Heavy metals. Zinc, lead, and nickel from cheap equipment fittings, uncoated metal clips, or contaminated salt mixes. Always use reef-safe, plastic-coated equipment inside the tank.
Household chemicals. Aerosol sprays (air freshener, cleaning products, insecticide) near the tank dissolve into the water surface. Fumes from fresh paint, new carpet outgassing, or cooking sprays can all contaminate a reef tank in enclosed rooms.
Stray voltage. Electrical current leaking from faulty equipment (heaters, pumps) into the water causes chronic stress that manifests as poor polyp extension and gradual bleaching. A grounding probe and regular voltage testing prevent this.
Nicotine. Hand contamination from smokers handling tank equipment introduces nicotine, which is toxic to invertebrates at very low concentrations. Always wash hands thoroughly before working in a reef tank.
Diagnosing Contamination
Contamination bleaching has a characteristic pattern: multiple coral species across different positions in the tank bleach simultaneously without an obvious temperature, light, or parameter change. When corals in both high and low positions, in both strong and weak flow, all begin paling at the same time, a waterborne contaminant is the most likely cause.
Run fresh activated carbon immediately. Perform a 25 percent water change with clean, tested salt mix. Remove any suspect equipment. Test for copper if a test kit is available. If the contaminant is removed, corals typically begin recovering within 1 to 2 weeks.
Biological and Pathogenic Causes
Not all bleaching comes from environmental stress. Biological agents can trigger localized or colony-wide bleaching.
Bacterial Infection
Stressed corals are vulnerable to bacterial infections that can cause tissue loss and localized bleaching. Vibrio species are commonly implicated. Infection often follows physical damage (a broken branch, a fish bite, abrasion from falling onto rock) that creates an entry point for bacteria.
Infected areas show rapid tissue loss with a distinct boundary between healthy and affected tissue. This is different from environmental bleaching, which tends to be more uniform across the colony.
Coral Pests
Acropora-eating flatworms (AEFW), Montipora-eating nudibranchs, and red bugs all damage coral tissue in ways that can cause localized bleaching. The bleaching follows the pest's feeding pattern, often appearing as irregular pale patches or trails on the colony surface.
Inspect affected corals closely with a magnifying glass. Pests are often tiny (1 to 3 mm) and well-camouflaged against the coral surface. Dipping new corals in pest treatment before adding them to the display prevents most introductions.
Coral-on-Coral Aggression
Sting damage from neighboring corals causes localized tissue damage that appears as bleached patches at the contact point. This is often misidentified as disease or environmental bleaching. The pattern is diagnostic: bleaching occurs only on the side facing the aggressive neighbor and nowhere else on the colony.
Check for sweeper tentacle contact at night. Move the affected coral out of reach and the bleached area typically recovers within 2 to 3 weeks.
Diagnosing the Cause
When you notice bleaching, identifying the trigger quickly determines whether the coral recovers or dies. Work through this diagnostic sequence.
Step 1: Check Temperature
Is the current temperature within 76 to 79°F? Has there been any spike or dip in the last 48 hours? Check your heater, chiller, and controller. Review temperature logs if your controller records them. Temperature is the most common cause and the easiest to verify.
Step 2: Review Recent Light Changes
Did you upgrade fixtures, move the coral, rearrange rockwork, or change your light schedule recently? Any change that increased light exposure at the coral's position could trigger bleaching within days.
Step 3: Test Water Chemistry
Test alkalinity, calcium, pH, and salinity. Compare to your baseline. A significant deviation (alkalinity more than 1.5 dKH from normal, salinity more than 0.002 off, pH below 7.8 or above 8.6) is a likely contributor. Also test nitrate and phosphate. If either has dropped dramatically recently, nutrient crash bleaching is possible.
Step 4: Check for Contamination
Have you used any new equipment, medications, or cleaning products near the tank? Is there new paint, carpet, or construction in the room? Smell the surface water. Unusual odors suggest contamination.
Step 5: Inspect for Pests and Aggression
Examine the bleached coral closely with magnification. Look for tiny flatworms, nudibranchs, or bite marks. Check at night for sweeper tentacle contact from neighboring corals.
Step 6: Evaluate the Pattern
- Whole-tank bleaching: Temperature, contamination, or severe parameter swing
- Multiple species, one area: Localized light or flow problem
- Single colony, one side: Aggression from neighbor or pest damage
- Single colony, uniform: Specific sensitivity to a parameter change that other corals tolerate
Recovery
Bleached corals can recover if the stress is removed and conditions are stabilized. The speed and completeness of recovery depend on the severity of bleaching and the coral category.
Stabilize First
Before anything else, identify and correct the bleaching trigger. Recovery cannot begin while the stressor persists. Stabilize temperature, reduce light if it was the cause, correct water chemistry, or remove contaminants.
Reduce Light
Bleached corals have lost most of their photoprotective pigments along with their zooxanthellae. The tissue that remains is highly vulnerable to light damage. Reduce light intensity by 20 to 30 percent over the affected area or move the coral to a shaded position.
Do Not Overfeed
Without zooxanthellae to process photosynthetic products, a bleached coral's metabolism is disrupted. LPS with large polyps can benefit from very light target feeding (small amounts of amino acids or fine particulate food). SPS have limited ability to feed heterotrophically and should not be fed directly during bleaching recovery.
Wait
Zooxanthellae recolonization is a slow process. Residual algae cells within the tissue multiply gradually, and the coral may acquire new zooxanthellae from the water column. Initial signs of recovery (faint color returning to the tissue) may take 2 to 4 weeks. Full recovery to normal coloration can take 2 to 6 months.
SPS recover more slowly than LPS and soft corals due to their thin tissue and minimal energy reserves. A partially bleached SPS has a reasonable chance of recovery. A fully bleached SPS rarely survives unless it is a particularly hardy species (Montipora, Pocillopora).
LPS and soft corals recover more reliably. Their thick tissue provides energy reserves that sustain the coral through weeks without photosynthetic income. Many LPS survive full bleaching episodes and regain color over 1 to 3 months.
Common Myths
"Bleached coral is dead coral." Bleaching is not death. The coral tissue is alive but stressed and energy-depleted. Many bleached corals recover fully if the stressor is removed and conditions stabilize. Write off a coral only when the tissue is physically gone, not when the color is.
"Only SPS corals bleach." All photosynthetic corals can bleach, including LPS, soft corals, and even zoanthids. SPS bleach more easily and recover less reliably, which creates the impression that bleaching is an SPS-only problem. It is not.
"Coral bleaching is always caused by high temperature." Temperature is the most common cause, but light shock, parameter swings, contamination, nutrient crashes, and pest damage all trigger bleaching through the same oxidative stress pathway. Diagnosing the actual cause rather than assuming temperature is essential for effective response.
"You should increase feeding to help bleached coral recover." Aggressive feeding during bleaching can overwhelm the coral's reduced metabolic capacity and promote bacterial growth on stressed tissue. Light, infrequent supplemental feeding (for LPS with capable polyps) is appropriate. For SPS, stable conditions and patience are more effective than feeding.
"Once a coral bleaches, it will always be weaker." Corals that recover from bleaching can return to full health and coloration. The recovery process may even produce corals with zooxanthellae strains better adapted to the tank's conditions, resulting in greater resilience to future stress. A recovered coral is not permanently compromised.
FAQ
How quickly does bleaching happen?
Acute bleaching (from a temperature spike or light shock) can become visible within 24 to 48 hours. Chronic bleaching (from prolonged moderate stress) develops over 1 to 3 weeks. The faster the bleaching onset, the more severe the trigger.
Can I save a fully bleached SPS coral?
It is possible but difficult. Fully bleached SPS have very limited energy reserves and a narrow survival window (1 to 3 weeks before tissue begins dying). Move to a shaded area, stabilize all parameters, and wait. Montipora and Pocillopora have the best recovery odds among SPS. Fully bleached Acropora rarely survive.
Should I move a bleaching coral to a different tank?
Only if the stressor in the main tank cannot be quickly corrected. Moving adds handling stress. If you can resolve the cause (fix temperature, remove contaminant, adjust light), leaving the coral in place with reduced light is usually better than the additional stress of transfer.
Does coral bleaching affect fish?
The bleaching event itself does not directly harm fish. However, the conditions that caused bleaching (temperature spike, contamination, parameter crash) may also stress fish. Monitor fish behavior and test water chemistry to address the root cause.
How do I prevent bleaching during a power outage?
Battery-powered air pumps maintain oxygen exchange. Insulating the tank (wrapping in towels or blankets) slows temperature change. Avoid opening the tank lid, which accelerates heat loss in cold weather or heat gain in warm weather. For tanks in extreme climates, a UPS or generator for the heater and return pump provides the most reliable protection.
Is partial bleaching a sign of a bigger problem?
Partial bleaching (one side of a colony, one coral among many) is often localized and caused by aggression, a specific light angle, or minor flow issue. It is less concerning than whole-tank bleaching. Investigate the local cause and adjust. Partial bleaching usually resolves quickly once the trigger is corrected.