Figuring out what is wrong with your coral can seem like a daunting task…


Fish are much easier to diagnose because they exhibit many different physical and behavioral symptoms. But corals just sit there and look very sad indeed. In fact, a lot of coral problems simply resemble coral tissue turning into slime to the untrained observer.


Did you know that pests, predators or diseases will NOT actually cause most of the coral problems most saltwater hobbyists encounter…

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what is wrong with your coral?

“Success With Corals” teaches you how to find out what is wrong with your coral and fix it! (photo credit: Stephane Duquesne)

Most coral problems are caused by poor water quality that will most likely go away by themselves when conditions are made more favorable


This is good news! The most common causes of coral problems are:

  1. Excess nitrates
  2. Excess phosphates (and more rarely ammonia or nitrites; although these 2 chemicals should not be present in an established aquarium with a adequate biological filter set-up)
  3. Fluctuating (meaning constantly shifting, so not stable) temperature
  4. Fluctuating pH
  5. Inappropriate lighting

These all cause corals distress and stress, if you have coral problems you should check these parameters immediately and correct if necessary.


Poor water quality and coral disease

Poor water quality will cause stress, which can be a causative agent of disease too.

For example: the coral becomes stressed out dealing with poor quality water, then the corals natural defense systems fail to protect it from ailments that a healthy coral can easily fight off (like a human cold).

So don’t just treat the symptoms, look for the cause!

This is another reason a detailed water quality analysis should be carried out as soon as you witness any coral problems to get to the root cause of the problem. 90% of the time there is going to be a parameter out of balance instead of a naturally occurring pest, predator, parasite or disease in your saltwater tank.

reef tank problems

Most likely your coral problems are caused by poor water quality or lighting issues. (photo credit: Glen Marsch)

How inappropriate lighting affects your corals

Other than poor water quality there is another big issue that negatively affects a lot of corals in this hobby and that is inappropriate lighting.

This will result from either

  1. Aging bulbs putting out the wrong spectrum of light (the spectrum slowly changes as they age becoming increasingly unsuitable for corals).
  2. When a new coral is placed in an area where they get not enough or too much light.

A new coral can be positioned toward the bottom of its light “range” and be gradually moved up, staying in each position for several days to a week. Take a photo at each position, at the end the photos will tell you which position the coral is most expanded at; this will be your perfect placement!


So poor and/or fluctuating water quality and wrong lighting are the most common causes of stress; other common coral stressors are:

  • incompatible tank mates (coral-nipping fish/crustaceans and/or other stinging celled organisms attacking your coral)
  • starvation
  • physical damage
  • poor water flow


troubleshooting coral problems

Corals like this Hammer coral can go from being fine to very sick looking in a few hours…

Here are the common symptoms coral stress can result in:

1. Tissue recession: this can result from a number of forms of stress, even diseases and parasites. The corals soft tissues recede and die, they appear to be eaten away gradually from the coral skeleton. A band of advancing bacteria often marks tissue recession, confusingly this band could be the cause (pathogen) or the bacteria could just be feeding off the wasting tissue.

2. Lack of polyp expansion: this is when the polyps don’t fully extend (or come out at all) themselves out of their calcium carbonate skeleton casing. Sometimes the polyps may be constantly expanded in which case the coral is probably trying to gather more light because it is not getting enough. If this is occurring after you have made a change to the water the coral could be trying to get more nutrients or flushing itself out.

3. Slowing or stopping of growth: This is a common symptom when the stress is caused by not enough trace elements especially calcium, magnesium and strontium in your aquarium water.

4. Coral Allelopathy: this is basically a chemical turf war between corals, where toxic metabolites are released into the water to keep encroaching neighbor corals at bay. It is especially common in smaller, crowded tanks with many coral species. Soft corals such as Coralliamorphs and Gorgonians are usually the worst culprits of this toxic warfare with small polyped stony corals (SPS) getting highly stressed by this. Other stony corals and even anemones can participate in this chemical battle.

5. Bleaching of the coral: this is caused by the massive expulsion of photosynthetic zooxanthellae in the tentacles of day feeding (photosynthetic) corals, bleaching is the release, rejection, or loss of zooxanthellae from coral tissue. Often low numbers of zooxanthellae will remain in the coral tissue so many corals recover from a bleaching incident as the remaining zooxanthellae multiply when the stressor is removed. If bleaching has occurred the coral tissue often appears white (not always; some colours are from fluorescing proteins not zooxanthellae themselves) and the coral polyps may be expanded. As bleached corals are no longer able to photosynthesise they need to be target fed food (usually plankton) to survive this event, conditions should also be made favorable for zooxanthellae to repopulate coral tissues (and so the coral recover) by adding nutrients, namely dissolved nitrogen to the water column while the zooanthellae are re-establishing, this will speed up the process.

6. Starvation: this can be a direct result of bleaching but also occurs when corals aren’t getting enough light or when corals that require supplementary feeding (almost all corals will get benefit by feeding them plankton etc a few times per week) aren’t being fed enough to remain healthy, which many people do. Starvation can also result when there are not enough nutrients (nitrates/phosphates = plant growth nutrients) in the water for good zooxanthellae health, so your water can actually be too clean! When a coral is starving it will begin to feed on its own tissues so you will see tissue recession.


So, most coral problems come down to a few main things which cause stress. When you see something wrong with your coral you have to think about if anything has happened inside your tank in the last few days to change the environment. Then test all your parameters (over time if needs be) to figure out your underlying cause. Get rid of the cause and your corals will gradually come back to health.

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