Coral Bleaching in Your Tank: Causes Beyond Temperature

Ask why a coral turns white and everyone says the same word: temperature. Ocean documentaries have taught us all that bleaching means heat. But in home aquariums — where heaters and controllers hold temperature steadier than any natural reef — the majority of bleaching events have nothing to do with heat at all. Corals bleach in tanks with perfect 78F stability every day, and the real causes hide in light, chemistry, and nutrition. Understanding what bleaching actually is makes the whole diagnostic obvious.

What bleaching is (and is not)

A coral's color and most of its food come from zooxanthellae — symbiotic algae living inside its tissue. Bleaching is the coral expelling those algae under stress, leaving translucent tissue over white skeleton. Two crucial clarifications:

• A bleached coral is alive. The tissue is still there, still intact — just pale and starving. This is why bleached corals can recover.
• Bleaching is not tissue loss. A coral turning white from the base up while tissue peels or sloughs is dying of recession or infection, not bleaching. Look closely: bleached tissue is transparent but present (you can often see polyps still extending); recession leaves bare, algae-ready skeleton. The two problems have different causes and different fixes, and confusing them wastes your response time.

Also not bleaching: a coral going pastel under strong light (controlled zooxanthellae reduction that many SPS keepers deliberately chase), or a brown coral becoming more colorful as nutrients drop.

Cause 1: Light shock — the number one tank bleacher

The most common cause in home reefs, and it usually follows something the keeper did:

• New fixture, or the annual bulb/LED upgrade
• Cleaning a fouled lens or dusty light after months — instantly restoring 20-30% more PAR
• Moving a coral up the rockwork
• Removing a colony that shaded a neighbor
• Ramping a program to full after weeks of running dimmed

The pattern that gives it away: bleaching worst on the tops of colonies and the highest-placed corals, appearing within days of a lighting event. Recovery protocol: cut intensity 30-50% (or shade affected corals with screen), then walk back up over three to four weeks. Every new coral and every lighting change deserves deliberate acclimation — our PAR guide covers targets by coral type and why measuring beats guessing with a full-spectrum reef LED or any other capable fixture.

Cause 2: Alkalinity events

Stony corals bleach and burn from chemistry faster than most keepers believe. A dosing pump stuck on, a new salt mix batch far from your tank's baseline, a big water change with mismatched water, kalkwasser accidents — alkalinity spikes (or crashes) trigger tissue stress that shows as bleaching, burnt tips, or patchy whitening, often tank-wide and fast. If multiple stony corals whiten in the same week, test alkalinity before you touch a light. Run the full panel with a complete reef test kit and compare against your logged baseline — which is the argument for keeping a log at all.

Cause 3: Starvation — the slow bleach

Zooxanthellae need nitrogen and phosphorus. In aggressively filtered tanks — oversized skimmer, GFO, carbon dosing, too-few fish — nitrate and phosphate hit zero and corals slowly pale over weeks, starting with the most delicate SPS. This bleach looks different: gradual, tank-wide pastel fading without a triggering event, often with thin tissue and poor polyp extension. The fix is feeding the system: more fish food, reduced export, target nitrate 3-10 ppm and phosphate 0.03-0.08 ppm. Zero is not a trophy; it is a famine.

Cause 4: Temperature — real, but check the details

Heat bleaching absolutely happens in tanks: a failed heater stuck on, a summer heat wave with no fan, a controller probe drifted out of calibration. Sustained time above ~82-83F stresses most corals; spikes higher bleach them quickly. Cold does it too — a heater failure in winter dropping the tank below 72F. The tell is your log or controller history, which is why a temperature record matters more than any single reading. If you cannot rule temperature out, put a second thermometer in the tank today; drifted probes are common.

Cause 5: The miscellany that mimics bleaching

• Salinity swings from top-off failures or sloppy water changes
• Chemical contamination — metals from corroding equipment, aerosols, hand cream
• Dinoflagellate or cyano coatings suffocating tissue (looks dusty, not translucent)
• Stings from neighbors, bleaching one localized patch facing the aggressor

Single-colony whitening usually has a local cause (sting, shading change, pest); tank-wide whitening has a systemic one (light, chemistry, temperature, starvation). That one distinction, borrowed from our closed-coral diagnostic, aims your entire investigation.

The recovery playbook

Bleached corals recover if the stressor stops and the tissue gets fed:

1. Identify and remove the cause — everything else is secondary.
2. Reduce light 30-50% regardless of cause; pale tissue photodamages easily without its algae.
3. Feed the coral directly — bleached corals cannot photosynthesize enough, so target-feed LPS meaty foods and broadcast-feed fine foods for SPS and softies weekly.
4. Hold parameters boring — no experiments, no new dosing regimens during recovery.
5. Wait in weeks, not days. Color returns as zooxanthellae repopulate — typically three to eight weeks from pale to presentable.

If deep suspicion remains after the obvious causes test clean — contamination, trace metals — a mail-in ICP water analysis reads thirty-plus elements and has caught more than one mystery bleaching traced to a rusting clamp or contaminated salt batch.

FAQ

Can a fully white coral recover?

Often, yes — if tissue is intact and the cause is fixed. Recovery odds drop sharply once tissue recedes or algae colonizes skeleton. Shade it, feed it, keep water stable, and give it a month before judging.

Why did only one coral bleach?

Local cause: it got moved, a shading neighbor was removed, a pump shifted to blast it, or a sting from an adjacent colony. Tank-wide stressors bleach broadly; single-victim events are almost always about that coral's specific location.

Should I do a big water change when corals bleach?

Only if you have evidence of a chemistry problem or contamination. If the cause is light shock, a water change does nothing; if it is starvation, aggressive water changes make it worse. Diagnose first, act second.

Bleaching in a home reef is rarely mysterious — it is light, alkalinity, starvation, or temperature, in roughly that order. Find the change, reverse it, feed the coral, and be patient. The full prevention framework lives in our guide to coral care and propagation.