Free coral health tool

Coral Bleaching Simulator & Reef Tank Stress Checklist

Coral bleaching is when a stressed coral, most often from water that is too warm, expels the tiny symbiotic algae (zooxanthellae) living in its tissue, losing its colour and main food source so the white skeleton shows through. Bleached coral is not dead yet, but it starves if the stress lasts.

Interactive simulatorDegree Heating WeeksWhy is my coral white?Printable log

Coral bleaching simulator

Set the stress

Move the sliders and watch the coral and its algae respond.

+0.0 °C
Normal max
+4 °C
0 days

NOAA counts heat stress over a rolling 12-week window.

Steady, no change
Other stressors

Simulated coral

Illustrated branching coral: Healthyalgae: 14/14
  1. Healthy
  2. Stressed / paling
  3. Bleached, tissue intact
  4. Tissue loss / recently dead

Stage

Healthy

The coral and its algae are working normally. Temperature and light are inside what this coral is used to.

Degree Heating Weeks
0.0 °C-wk
NOAA-style alert
No heat stress
Algae left (illustrative)
100%

Water is at or below the coral's normal warmest temperature.

What the zooxanthellae are doing

Millions of zooxanthellae (symbiotic algae) live inside the coral's tissue. They photosynthesise and hand most of the sugar they make to the coral, and their pigments give the coral its brown, gold or green colour.

Outlook: No stress to recover from.

Conceptual model for learning, not a calibrated prediction. Real corals differ by species, history, and location. Heat stress follows NOAA's Degree Heating Weeks idea; the light and other stressor effects are illustrative weights.

Healthy vs. bleached vs. tissue loss

The question that matters most is whether tissue is still there. Colour can come back; bare skeleton can't.

Illustration: Healthy coral
NormalBrown or coloured tissue packed with zooxanthellae. Polyps extend; tips may be paler where the coral is growing.
Illustration: Bleached, tissue intact coral
Bleached, tissue intactAlgae expelled. Living tissue is see-through, so the white skeleton shows. Still alive and able to recover.
Illustration: Tissue loss / recently dead coral
Tissue loss / recently deadTissue has died and sloughed off. Bare skeleton is quickly covered by a brown-green film of turf algae.

Causes of coral bleaching

Reef-building corals depend on zooxanthellae, single-celled algae living inside their tissue that make food from light. When water stays too warm, the algae's photosynthesis breaks down and starts producing harmful by-products, so the coral expels them. On wild reefs, unusually warm water driven by climate change and marine heatwaves is the leading cause of mass bleaching (NOAA).

NOAA also lists strong sunlight during hot spells, storm rain and runoff that dilute seawater and carry pollutants, and air exposure at extreme low tides. Some pollutants, such as herbicides, can damage the algae directly (EPA). Several mild stresses together can bleach a coral that would cope with any one alone, which is why the simulator lets you stack them.

Degree Heating Weeks, explained

NOAA Coral Reef Watch measures heat stress in Degree Heating Weeks (DHW). Each week the water is at least 1 °C above the usual warmest-month temperature adds that many degrees; the total is kept over a rolling 12 weeks. Two weeks at 2 °C too warm equals 4 DHW.

Around 4 DHW reef-wide bleaching is expected, especially in sensitive species; by 8 DHW bleaching with death of heat-sensitive corals is likely, and from 12 DHW multi-species mortality becomes likely (NOAA Coral Reef Watch). The simulator uses these thresholds for heat; aquarium corals and individual species can be more or less tolerant.

Why is my coral turning white? Reef tank troubleshooter

Answer a few questions about your aquarium coral to rank possible explanations: bleaching from heat or light, tissue loss (RTN or STN), normal growth tips, light shock, low nutrients, chemistry swings or pests. Then print the checklist and log to track it.

Answer what you know

Skip anything you're unsure of. Results update as you answer.

1. Is living tissue still over the white area?

Look closely with a flashlight. Bleached tissue is see-through but soft and smooth, often with polyps. Bare skeleton is crisp, textured, and has no polyps.

2. Where is the white?
3. What kind of coral is it?
4. How fast did it happen?
5. Recent changes and water trends

Possible explanations

Answer the first questions, starting with whether tissue remains, to see possible explanations ranked by how well they fit.

Not a diagnosis. These are possibilities that fit your answers. Make one change at a time, correct parameters gradually, and keep conditions stable. There is no universal medication or dose for bleaching.

Reef tank stress observation checklist & parameter log

General checklist. Answer the questions above to tailor it. Print it and keep it by the tank.

  • Look closely with a flashlight: is translucent tissue or polyps still over the white area, or is it bare, textured skeleton?
  • Note exactly where the white is (tips, top, base, patches) and photograph the same spot daily in the same light.
  • Check polyp extension at night and record it.
  • Test temperature, salinity, alkalinity, nitrate and phosphate with calibrated kits or meters, and compare with the last 2 weeks of readings.
  • Write down every recent change: lights, schedule, placement, new livestock, equipment, salt or additives.
  • Check other corals: is one coral affected, or many?
  • Check the heater, chiller and controller logs for the highest temperature in the last 2 weeks.
  • Test alkalinity at the same time on several days to see the daily swing.
Parameter log
DateTempSalinityAlk (dKH)Ca (ppm)Mg (ppm)Nitrate (ppm)Phosphate (ppm)PAR / light changeCoral colour note
          
          
          
          
          
          
          
          
          
          
          
          
          
          

Log these parameters automatically in My Reef Log and see temperature, alkalinity and nutrient trends on charts instead of paper.

Can bleached coral recover?

Yes, if the stress ends in time. Bleached tissue is alive, so once temperature and light are back to normal the coral can take up algae again and regain colour over weeks to months. Recovered corals often grow and reproduce less for a while and are more prone to disease. In a tank, fix the cause, keep parameters steady, and avoid piling on changes.

Is white coral dead?

Not necessarily. Look closely: bleached coral has smooth, translucent tissue and often extended polyps. Dead coral is bare, crisp skeleton with visible cup-like pits and no polyps, and it grows a brown or green algae film within days. On SPS, white tips with polyps are usually new growth, not bleaching.

Reef tank vs. ocean bleaching

Ocean bleaching is mostly driven by long marine heatwaves over whole regions. In an aquarium you control the water, so causes are more local and faster: a stuck heater, a new light at full power, a salinity or alkalinity swing, nutrient starvation, or a new coral meeting brighter light than it grew under. That also means you can usually remove the stress quickly.

How to use the troubleshooter

  1. Step 1

    Check for tissue

    Look closely at the white area with a flashlight to tell see-through living tissue from bare skeleton.

  2. Step 2

    Note the pattern

    Record whether the white is on growing tips, the top, the whole colony, the base, or in patches, and how fast it appeared.

  3. Step 3

    Review recent changes

    List temperature, light, placement, new livestock, and alkalinity, salinity, nitrate, and phosphate trends from the last two weeks.

  4. Step 4

    Stabilise and log

    Fix the likely cause gradually, keep everything else stable, and log parameters and coral colour daily on the printable checklist.

Catch temperature and alkalinity drifts before your corals do: log tests in seconds and see trends in My Reef Log.Start tracking free

Sources

The simulator is a conceptual teaching model, not a calibrated health prediction. The troubleshooter suggests possibilities, not a diagnosis, and never replaces a close look at your coral and your test results.

Coral bleaching FAQ

What is coral bleaching?

Coral bleaching is when a stressed coral expels the symbiotic algae (zooxanthellae) that live in its tissue and give it colour and most of its food. Without them the tissue turns see-through and the white skeleton shows. Heat is the main cause on wild reefs.

Is white coral dead?

Not always. A bleached coral is white but alive: thin, translucent tissue still covers the skeleton and polyps may still extend. Coral is dead where the tissue has gone and only bare, textured skeleton is left, which turns brown or green with algae film within days.

Can coral recover from bleaching?

Yes, if the stress ends before the coral starves. Bleached corals can take algae back up and regain colour over weeks to months. The longer and hotter the stress, the more likely corals die instead; NOAA links 8 or more Degree Heating Weeks with reef-wide bleaching and death of heat-sensitive corals.

What causes coral bleaching in a reef tank?

In aquariums the usual causes are a temperature spike (a stuck heater or hot room), too much light too fast (a new fixture, higher intensity, or moving a coral up), big salinity or alkalinity swings, acclimation stress in new corals, and nitrate and phosphate sitting at zero for weeks.

Why is my coral turning white?

First check whether tissue remains. White tips on fast-growing SPS are often normal new growth. An evenly pale colony with tissue points to bleaching from heat, light, or low nutrients. Bare skeleton spreading from the base or in patches points to tissue loss (RTN or STN) or pests. Use the troubleshooter above to rank the possibilities.

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