Over the years I have been asked a lot of questions by hobbyists regarding their corals and their tanks. Of these queries, the single most frequently asked question is” Why are my corals dying or why did this coral die?” Obviously my first response is: “what are the parameters in your tank”. Invariably the response is the parameters are perfect, or at least reasonably close.

My next question is: “Is there anything in your tank that could be eating or is predatory to your corals?” Here the response is usually, no there is nothing in there that could be doing this. So then I ask in a joking manner: “So I guess your corals committed suicide?” Life in your tank is so bad that your corals decided to end it all?

Some get that I am teasing, but some think that this is indeed the case. I know this is not a profound statement, but, corals do not die for no reason. Some times the reason is obvious, but a lot of time it takes a lot of effort to eventually find the cause. Unfortunately I have lost many corals over the years for what I deemed to be ‘no reason’ but now realizing that is not plausible, I have been able to find many of the causes, many of which I would not have thought possible.

The first thing I tell people who are losing corals is to do a complete test of the tank’s parameters repeatedly over a week’s time. Also when they do this I suggest that they look at the expiration dates on their test kits. If things are badly out of whack this is easy to see and fix, but this is rarely the cause. However, there is one simple test that I have now seen happen in at least 10 tanks, all of which were losing corals for seemingly no reason.

In each instance the cheap box hydrometer they were using were significantly off and as a result the salinity in their tanks was significantly higher than what the hydrometer was indicating. Most of these tanks that were losing corals for this reason had salinity in excess of 1.030 or higher. Needless to say this salinity is usually not conducive to the long term health of most corals. As a result, I now use two refractometers, which I calibrate regularly using Julian’s AccuraSea reference solution. Since many of us now use refractometers to measure our salinity the problem of hypersalinity is not as much of a problem as it used to be, but it still can happen.

After testing is done, the next thing I suggest is to watch the tank in the middle of the night. I further suggest getting a flashlight that emits a red beam of light for observing the tank at this time. Most marine organisms don’t see in red light, but we can, so using this lights enables us to see what is going on in the tank without causing the animal’s behavior to change. No pun intended, but it was a real eye opener when I started using this light at night. I saw several animals that I did not even know were in my tank, eating and damaging the corals.

The first animal that I did not know was in my tank as a black-clawed crab that was approximately the size of the palm of my hand. It had undoubtedly come in as a small stowaway on a piece of rock and over time hand grown into this monster. When I saw it it was munching away on a piece of coral, obviously I removed it. Even after removing it I still continued this practice, especially if I was losing corals or seeing damage I continued this middle of the night reconaissance.

Doing so I also found several fire worms and large bristle worms that at night were actually either eating the tissue on the Euphyllia or going into the mouths of these corals to extract food and damaging them that way. I had origianlly blamed it on my Moorish Idol for causing this damage, but it was these worms.

I also plucked out multiple Asterina starfish that came out at night to eat Acropora tissue as well as multiple pairs of blue-eyed crabs that were eating coral polyps. And I even saw an urchin that was eating the growth tips of Acropora at night, that didn’t bother anything during the day. None of these animals were causing massive damage at any one time, but each was at least partially responsible for damaging or stressing the corals enough to cause their demise for seemingly an unknown reason.

If I had not visited my tank regularly at night I never would have believed that all of these animals were present causing such damage. However the most amazing animal that I encountered on one of these nightly vigils was a large cone-shelled mollusc, that I believe was a whelk, but I am not a mollusk expert, that was almost 2 inches tall and where it’s mouth and radula were was the diameter of a quarter.

I had mistakenly always thought that this was just another snail. However when watching the tank one night I quickly realized that this was no ordinary snail in that, as it moved down the branch of an acropora, the only thing left was a bright white skeleton where it had eaten all of the tissue off of the coral .

This animal was the cause for the numerous “bleached” areas of my Acroporas that were seemingly doing so for no reason. Luckily this was the easiest pest to remove as it was large and slow moving. This is just a short list of pests that can wreak havoc in our tanks that can be found at night. I am only listing these as they are ones I have encountered that were not as readily apparent as some other pests and were only seen late at night.

All of these pests undoubtedly came in as unwanted hitchhikers, hence why the need for dipping and quarantine for virtually anything we add to our tanks. However, even when this is done and the tank is observed including at night, corals can still mysteriously die for seemingly no reason. Again there is a reason, we just need to find it.

Even after doing all of this and taking precautions with anything that was added to the tank, some times we add fish or other animals that we think are harmless or even beneficial. In my case I added six black clown gobies when I was battling flatworms and predatory tiny black brittle stars. These fish did an amazing job of eliminating these Acropora parasites. However, over time an interesting phenomenon developed. Every month or two there would be significant damage at the base of an Acropora. And for some reason it would always be on new corals that were placed in the vicinty of corals that had been damaged in the same spots earlier.

Over time I finally realized what was happening. At dusk, these gobies would attempt to spawn. But before they would span they would clear a spot in the Acropora colonies where they would lay their eggs. While most of the time these fish were great protectors of the Acropora, when they went into spawning mode they were critically damaging and killing their hosts. Once these fish were removed this damage stopped. Once again the corals were dying or being damaged for what seemed to be an unknown reason.

While the above listed causes for a coral’s demise for seemingly no cause occur far more often than we would like to admit, coral diseases are undoubtedly another frequent reason that we lose corals. I’m not going to bore you with a listing and description of these as I am clearly not an expert on coral diseases. However, the two most common diseases that almost everyone who has kept an sps coral has encountered are rapid and slow tissue necrosis, aka RTN and STN. When corals die from these it also is seemingly for an unknown reason.

In my tanks, and in the tanks of people where I have seen this happen the tanks for the most part tested well and had otherwise adequate conditions, so the cause of the outbreak was not readily apparent. Fortunately as Dr. Craig Bingman wrote in 1997 we understand that the cause for these maladies is bacterial. This bacteria is probably Vibrio and is present in low levels in most of our tanks right now, and as such does not cause many problems.

However, if we stress the corals by overheating the tank or overcrowding it or having inadequate water flow these bacteria can overgrow at an alarming rate and quickly lead to the demise of many sps corals. Corals where this bacterial overgrowth is occurring can succumb to these bacteria even if the conditions in the tank are otherwise perfect. So even in this case corals are not dying for an unknown reason.

After being on collecting trips and seeing corals go from reef to tank, I am amazed at how hardy and resilient most corals are. I am also amazed at the strides we have taken in growing and maintaing them long term. However with this success has come the realization that there are countless pests that will do our charges harm if we are not constantly vigilant, now seemingly 24 hours per day. While in the past I would have agreed with the notion that my corals died for seemingly no reason, I now feel better knowing that in just about every instance there was indeed a cause and if I do my homework I can find a remedy to prevent it from happening again.