Coral Bleaching or Coral Dying - Why?
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Coral Bleaching or Coral Dying - Why?

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Last updated on November 17th, 2023


Coral Bleaching or Coral Dying - Why?

Why are my coral bleaching / coral dying? My parameters are fine.

When combing through Facebook groups and a variety of forums, I almost always come across the same question. I'm sure you've seen it too. It starts with a picture of a beached or dead coral. Followed by "the question". Why are my coral bleaching? or Why are my coral dying? A perfectly valid question. Followed by the almighty line... "My parameters are good".

Not to say this isn't true. I'm sure, at that point, the parameters you measured are fine. There is a lot not seen here though. What other details should you add? How about the parameters you're testing? We're going to dive in to how to figure our why your corals are bleaching or dying.

Parameter Levels

Though there are always success stories that step outside of the "acceptable" ranges, most success stems from within the below parameters. These parameters are based on the values given by Marine Depot. There are some changes from additional research, but most is original. Though these are the target values, as you will read later, stability is more important.

Temperature76.0 F82.0 F
Salinity1.023 (31ppt)1.026 (35ppt)
Calcium350 ppm450 ppm
Magnesium1200 ppm1400 ppm
Nitrate1 ppm5 ppm
Phosphates0.0625 ppm0.3125‬ ppm
Ammonia0 ppm0 ppm
Nitrite0 ppm0 ppm

Read more about the Best Water Parameters for Reef Tanks here.

Aquarium Parameter Swings

If your parameters change too much in a short of time, you will be in a much worse state than your levels slowly shifting out of range. This is very similar to the idea of how to boil a frog. If you change temperature over time, the frog won't even realize what's going on. On the other hand, if the temperature increases quickly, the frog will panic. Coral have similar behaviors. Changes over time will not have nearly as much affect as a sudden parameter jump or drop.

This works for both the problem arising as well as fixing it. Let's look at an example.

Aquarium Parameter Swing Example

If your calcium suddenly jumps it will cause stress on the coral. This stress alone can lead to coral bleaching, or even coral dying. Now to bring this level down, most people panic. Large water changes, dosing alkalinity buffers, etc. In doing this, you've introduced a sudden change in that calcium, as well as the other aquarium parameters. This jumps the stress up again. Leading to more coral bleaching and more coral dying.

Though it may be appropriate to do large water changes for some parameter spikes. One example being ammonia or nitrites present in the aquarium. In the case of alkalinity, calcium, and magnesium, it's best to just monitor the levels and let them decline naturally. This happens through coral consumption and small water changes. If the parameter continues to go up, or they are really high, stop dosing. The levels will slowly adjust themselves. Again, there are exceptions. If your calcium is over 800ppm, you may want to do a couple 10% water changes over the course of the week. This will bring it down to the 600s where the coral consumption and normal water changes will heal the problem.

Nitrates and phosphates are another good example. Large changes often lead to other issues and coral death or coral bleaching. Solving the source of the nitrates and phosphates is much better. Adding a refugium, using filter socks, or even using GFO are all slower, more stable options.

Untested Parameters / Contaminates

Let's assume that your testing yielded no results. All values, both before and after the coral bleaching and coral death, were good and stable. Another worry is that some other chemicals got in the tank. Some things like copper can be tested. Others, like trace amounts of chlorine or excess minerals are much harder to detect. The simple solution is to run ROX 0.8 Activated Carbon.

Activated carbon is used in RO filters as a means to remove contaminates from the water. Though it is not a guaranteed solution, it's a fast, cheap attempt. The ROX 0.8 Activated Carbon is the top of the line option and is still very affordable.


Running out of ideas? What about your lighting? Lighting that is too weak, or even too strong, can cause coral bleaching and coral death. This is most common in SPS corals, but can also occur in LPS.

For more information on lighting, check out these articles:

Miscellaneous Causes

You should also look for other, less obvious causes. Common examples are stinging from anemones or corals. Close proximity to anemones and corals such as torch coral can be dangerous for most coral. The coral will be stung and eventually bleach or die.

Other things to look for are pests such as nudibranchs, flat worms, and red bugs. These are all commonly known to live on/near coral as well as eat them. Aiptasia can cause stinging just like any other anemone.

If you find a pest, look at the Reef Stable Pest Solutions Center for cures!

The ideal post

Still nothing obvious? Well, sometimes this happens to all of us. There can always be unknown or less obvious culprits. In that case, it's time to take to Facebook, Reddit, or forums such as Reef2Reef. But what should you post? Let's look at an ideal example.

  • State the issue you're having with full detail.
  • Size of your tank.
  • Parameters before and after.
  • Lighting do you use
  • Any recent changes (additions/removed)
  • Add a photo of the problem.


Hello, I have found that my SPS coral have been dying and I'm unsure of why. I have a 120g aquarium (24 in tall) and the acros are 8 inches from the top. My parameters are: Temp 78 Alk 9dKh Calcium 425 Nitrates 5 Phosphates 0.09 The Alk 2 days ago was 9.6. Recently added another acro about 6 inches away. I have 2 Radion XR30 Pros running at 80% and 2 MP40s for flow. Current fish are clownfish, gobies and cardinals. Any ideas on why my old acros are dying?

This seems hopeless, but you will likely hear much less about your parameters in the comments and a lot more about pests, which is the likely cause in this case.


I know, that was a lot. There are many reasons for coral bleaching and coral dying to occur. Hopefully this guide helped to shine some light on things to look for. When all else fails, please utilize the detail about an ideal post to get information that is targeted to your concerns. Best of luck!

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About the Author

Reef Stable Founder John Krenzer

John is a Software Engineer with a passion for saltwater aquariums, as well as the founder and president of Reef Stable. He started in the aquarium hobby as a child with a 20 gallon freshwater aquarium. His interest in aquarium life grew and in 2008, John set up his first saltwater aquarium.

Today, John maintains an over 300 gallon reef tank system, consisting of a 120g reef and a 210g reef. These large tanks are contained within the same system, sharing a sump as a means to reduce total maintenance and increase total water volume.

John writes articles for the blog as a means to learn about more reef aquarium topics. These articles act as a reference for the readers as well as himself. John updates these articles frequently to provide additional information or make corrections as new information becomes available.

If you would like to request an article, tank tour article, or to collaborate, let me know via the Contact Me Page!

Frequently Asked Questions

How does bleaching kill coral?

Coral bleaching is when the microscopic algae, zooxanthellae, leaves the coral. The coral will begin to turn white as the zooxanthellae is what provides the coloration. When this leaves the coral, the coral loses the ability to eat/consume light.

What is coral bleaching?

Coral bleaching is when the microscopic algae, zooxanthellae, leaves the coral. The coral will begin to turn white as the zooxanthellae is what provides the coloration.

What causes coral bleaching?

Though there are many causes for coral bleaching, they all revolve around coral stress. The most common are sudden changes in light, high water temperatures, and sudden changes in water chemistry.

Can coral bleaching be reversed?

It is possible for the coral to survive, grow, and produce zooxanthellae. It is generally quite uncommon, and requires removing the causes of the stress without adding more stress on the coral. It is very rare for coral bleaching to be reverses.

About Reef Stable

Reef Stable was initially founded in 2019 as a reef tank parameter log to fill a need. Reef Stable quickly grew, becoming a location to solve all of your reef tank problems as well as a place to learn.

Reef Stable now provides a Reef Blog, Reef Aquarium Guides, Coral Care Guides, Identification and Solutions for Pests and Algae, and Reef Dosing Calculators, in addition to the original Reef Parameter Log.

Reef Stable continues to grow, striving to provide a single location for all your reef tank needs!

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