What Causes Coral Bleaching in Reef Tanks?

Coral bleaching happens when corals lose their colorful symbiotic algae (zooxanthellae) due to environmental stress. The coral doesn't die immediately, but without these algae providing nutrients through photosynthesis, bleached corals will eventually starve if conditions don't improve.

Common Causes of Coral Bleaching

Temperature Stress

Temperature is the most common trigger for bleaching. Both excessively high (above 82°F) and low (below 74°F) temperatures can cause corals to expel their zooxanthellae. Sudden temperature swings of more than 2-3°F in a short period are particularly stressful.

Prevention:

• Maintain stable temperatures between 76-80°F
• Use a quality heater with a backup
• Consider a chiller if your tank runs hot
• Track temperature trends with the ReefBay app

Lighting Issues

Too much light intensity or sudden changes in lighting can shock corals into bleaching. This often happens when upgrading lights or placing corals too high in the tank.

Prevention:

• Acclimate new corals to your lighting gradually
• When upgrading lights, reduce intensity initially and increase over 2-4 weeks
• Match coral placement to their lighting requirements

Water Quality Problems

Poor water quality—including high nitrates, phosphates, or low alkalinity—stresses corals and can trigger bleaching.

Key parameters to maintain:

• Alkalinity: 8-11 dKH (stability matters more than exact number)
• Calcium: 400-450 ppm
• Magnesium: 1250-1400 ppm
• Nitrates: Below 20 ppm (ideally 5-10 ppm)
• Phosphates: 0.03-0.1 ppm

Chemical Contamination

Copper from medications, cleaning products, or even new equipment can poison corals. Heavy metals and certain chemicals cause rapid bleaching.

What to Do If Your Coral Bleaches

1. Don't panic – Bleached corals can recover if action is taken quickly
2. Test all parameters – Identify what's out of range
3. Check temperature – Ensure it's stable at 76-80°F
4. Reduce lighting – Lower intensity by 25-50% temporarily
5. Improve water quality – Do a water change if parameters are off
6. Feed the coral – Target feeding can help stressed corals recover

With prompt attention to the underlying cause, many bleached corals can recover their color within weeks to months. Monitor your parameters consistently with the ReefBay app to catch problems before they cause bleaching.