The Reefer: Clam and Coral Bleaching, Part 2: What to Do if It Occurs in Your Aquarium

Author: James W. Fatherree, MSc

There are numerous possible reasons for clams and corals to bleach, and there are a number of common ones that you can actually do something about, too. But, before we get into that, let’s have a quick recap of last month’s column, which covered the various causes and types of bleaching.

What is Bleaching?

Under some conditions, tridacnid clams and reef-building corals that contain symbiotic algae (zooxanthellae) may become pale in color, or even turn completely white. This loss of color is called bleaching, and it can affect only some areas of a clam or coral, or it can affect a whole individual or colony in the worst cases.

Basically, bleaching is a lightening in color or complete loss of color that results from the loss of some (or all) of a clam/coral’s zooxanthellae and the various pigments associated with them. Their colors that we see come from various pigments in the zooxanthellae and other pigments that the host animal produces, so when these are reduced in quantity for any of a variety of reasons, a host can lighten up and even fade out.

Causes of Bleaching

There are several things that can cause this to occur at some scale or another, but over-illumination (light shock) and/or unacceptably high temperatures are the primary culprits in aquariums. These are the two primary causes of bleaching in the wild, too. Still, tridacnid clams and corals can also bleach as a result of under-illumination, or being kept in water that isn’t warm enough.

Then there’s bleaching due to a lack of required nutrients, unacceptable salinities, poisoning by metals, too much red light, the use of some medications, or being over-stressed for some reason. (ex. Duquesne & Coll 1995, Brown 1997, Braley 1998, and Borneman 2001). And, as if that’s not a long enough list, the bleaching of some corals can be disease-related, too (Kushmaro et al. 1996 and Rosenberg & Loya 1999), and was also suggested by Norton et al. (1995) as a probable cause for some cases of tridacnid bleaching. In other words, there are lots of things that can cause bleaching.

Causes and Solutions

With that said, how about some good news? If you can figure out what is causing a clam or coral to bleach and provide a remedy, there is a great chance that they can recover. In fact, Leggat et al. (2003) reported that during a large-scale bleaching event in Australia over 70% of 9000 tridacnids bleached and appeared totally white, but that 95% of them also survived. After a recovery period of 8 months, the previously bleached clams were actually indistinguishable from those that did not. Good news, indeed. While corals often fare much worse, many can pull through if conditions are improved, as well.

So what do you need to do if something (or everything) bleaches in your aquarium? Well, since there are so many things that can cause it, it can sometimes be pretty tough to put your finger on a specific reason for it to occur in an aquarium and fix just that one thing. However, by starting with the basics, and using your head, there’s a good chance you can find and fix the problem.

Water Quality

To start, it is imperative that your water quality is up to par, and you should immediately correct anything that is outside of the acceptable ranges. The salinity should be about 35 ppt, which equals a specific gravity of around 1.025 measured at 25°C (77°F), if you use a hydrometer. The pH should optimally be between 8.1 to 8.3, but a pH of 8 to 8.4 is acceptable.

Alkalinity should be a minimum of 2.5 meq/L (about 7 dKH), with a value somewhere between 3.2 meq/L (9 dKH) and 4.3 meq/L (12 dKH) being optimal. And calcium levels should optimally be between 380 and 450 ppm, and no less than 360 ppm. Ammonia should also be less than 0.1 ppm, and nitrate is best kept well below 10 ppm, with values less than 1 ppm being optimal.


Then there’s the water temperature, which is best kept in the range of 77° to 82°F. Many hobbyists may run their tanks a little higher or lower than this, but sometimes temperatures can drift well out of this range and cause real trouble. Elevated temperatures are common in reef aquariums due to the use of high-intensity lighting, and when summer comes along and the AC quits, temperatures can easily rise too high.

Again, this is one of the most common causes of bleaching. Thus, if something in your tank bleaches due to unacceptable temperatures, you need to do whatever it takes to get into that 77° to 82ºF range as quickly as possible. Even if your tank usually runs higher, I think you’ll be better off reducing the temperature to 82°F or less, at least until everything has recovered (if it does).


The next thing is deciding if a clam/coral is getting too much light, especially UV light. If you’ve gotten a new specimen of some sort and it starts to bleach after you’ve acclimated it to your aquarium, and you feel confident that it’s not due to any of the things above, then you may not have given it adequate time to fully adapt to your lighting. Clams and coral can adapt to more/less intense lighting to some degree, but it takes time.

You also won’t know what sort of lighting the specimen was been living under before it arrived at a store, so there’s a possibility that your lighting may actually be significantly brighter than what it’s used to. If it has been living under lower-light conditions for some time and you plop it under some really bright aquarium bulbs, you just might overdo it (which is often called “light shock”) and cause it to bleach.

Reducing Light

The easiest thing to do in such a situation is to use one or more sheets of screening material to reduce the light the specimen is receiving. This stuff is available at most any hardware/home improvement stores, and you can cut out pieces of material that shade the specimen in trouble by placing it on a glass top, or using clothespins to affix a strip over the top of one area of the tank, without shading everything else in the tank that’s doing fine.

The best thing to do is to cut down the light using a few overlaid pieces of screen and then remove one piece at a time over a period of a couple of weeks. This will also allow you to leave the specimen wherever you placed it, rather than move it around over and over, which can be particularly bad for clams.

Changing Bulbs

Likewise, a specimen may be light-shocked when you change the bulb(s) in your lighting system. Bulbs get dimmer as time goes by, but if you replace them often then there shouldn’t be too much difference between the output of the old and the new. However, most of us keep bulbs running as long as possible, which means that when they do get replaced, there will be a greater difference between the output of the old bulbs and their replacements. Maybe even enough difference to lead to bleaching.

Thus, it may be prudent to use pieces of shade cloth for some period of time after a bulb change. How long exactly? No way to say for sure, as it would depend on what kind of bulbs you have and how long you wait to replace them. But, if a specimen starts to bleach shortly after a bulb replacement, then you can guess that the change was very likely the cause, and should take appropriate action.

Sudden Changes in Water Clarity

There’s yet another way to light-shock clams, too. Over time aquarium water can become increasingly yellow in color due to the accumulation of substances collectively called gelbstoff or gilvin. This stuff is produced primarily by the decay of organic materials, and it can absorb significant amounts of light (Bingman 1996). So if the concentration of these yellowing compounds is rapidly reduced, this means that the amount of light hitting a specimen can rapidly increase.

Reducing Concentration

Water changes can reduce the concentration of yellowing substances, and so can the use of activated carbon, and/or using a skimmer. Thus, you don’t want to do large, infrequent water changes that would instantaneously and significantly reduce these yellowing substances and potentially let enough additional light hit a specimen to cause it to bleach. Likewise, you never want to use large amounts of activated carbon and replace it infrequently as this could potentially lead to bleaching, too.

Skimmers, on the other hand, tend to be run continuously, so it is unlikely that simply running a skimmer will ever cause a dramatic change in the concentration of yellow substances. However, if you decide to add a skimmer where there wasn’t one before, or upgrade to a bigger/better skimmer, keep in mind that this may also quickly reduce yellowing compounds, as well.

Partial Water Changes

For this reason (and others), I always recommend doing regular partial water changes, and think it is best to do small water changes on a more frequent basis, rather than big changes every once in a while. The standard recommendation is around 25 percent per month. Likewise, if you choose to use carbon, it’s better to use a small amount of a quality brand and replace it more frequently (like every month when you do water changes), instead of using large amounts and waiting longer to replace it.

Shade Cloth

Whether you follow these recommendations or not, if you think that a specimen has started to bleach due to a sudden reduction in yellowing substances, try the shade cloth. Bleaching under these circumstances is still light shock, albeit due to other reasons, so you’ll need to reduce the light and hope that things get better.

Other Factors

Lack of Light

How about bleaching due to a lack of light? Bleaching due to under-illumination can occur as a result of either never having enough lights over your aquarium to keep a particular specimen healthy long-term, or it could happen if your lights were okay when they were new, but have gone too long without being replaced. As I said, they get dimmer over time, and if you wait long enough they just might get too dim.

In such cases the solution is obvious. You can either upgrade your lighting system so that it’s adequate, or get new bulbs to replace old ones. Be careful though, because if your lights are so dim that a specimen is starving/bleaching and you suddenly increase the intensity dramatically, you may end up making things worse rather than better (see above…).

Lack of Nutrients

How about a lack of nutrients? That’s an easy one too, as you just need to add some. In most tanks this should never be a problem, but if for some reason a specimen isn’t getting enough of what it needs nutrient-wise, you can easily add some appropriate sort of food. There are several quality products available that contain various sorts of plankton or plankton-substitutes, which can be used to help ailing specimens, and to feed perfectly healthy tanks, too.


Then there’s the possibility of disease. Unfortunately, if bleaching occurs due to an infection of some sort, there isn’t much that you can do about it. I say this because there wouldn’t be any way for a typical hobbyist to identify what type of microorganism was responsible for the bleaching of a coral, or especially a clam. So, you’d potentially have to treat a specimen in a quarantine tank (I never add drugs to a display tank) with a variety of antibiotics, hoping that one of them kills off the infection. To make things worse, a treatment could be successful, but you might not know it until several weeks later. The offensive microbes could be dead, but the color of the specimen would still take a while to return. Again, this would be a very difficult thing to treat considering the unknowns and efforts involved.

Travel Stress

Lastly, we get to stress-induced bleaching that may have nothing to do with any of the rest of these things. Getting bagged up and shipped across the world, or just from one side of the country to another can be very stressful for specimens, and they may respond by bleaching. An annoying fish can stress out a specimen, parasitic snails can stress clams, and even something as simple as an unacceptable current may have the same effect in some cases, even if everything else is fine. So, aside from everything covered above, keep an eye on a specimen to see if it is being bothered by obnoxious tankmates, or is being blasted by strong currents, etc., and remedy the situation immediately if you find anything wrong.

The Road to Recovery

In any case, regardless of what you decide is the probable cause, keep in mind that a clam or coral is highly stressed when bleached (with the exception of the patchy bleaching of clams covered in Part 1), and will very likely benefit from some added food(s). Particulate foods may increase a specimen’s chances of recovering, as it can help to make up for the lack of food produced by the zooxanthellae. So even if you usually don’t feed your tank, I’d recommend that you do so at least until everything seems to be healthy and looking good.

References and Sources for More Information

Borneman, E. 2001. Aquarium Corals: Selection, Husbandry, and Natural History. TFH/Microcosm Professional Series, Neptune City, NJ. 464pp.

Braley, R. D. 1998. Report to GBRMPA on results of research done under marine parks permit no.G92/137. Aquasearch.

Brown, B. E. 1997. Coral bleaching: causes and consequences. Coral Reefs 16:S129–S138.

Duquesne, S. J. and J. C. Coll. 1995. Metal accumulation in the clam Tridacna crocea under natural and experimental conditions. Aquatic Toxicology 32:239–253.

Fatherree, J. W. 2006. Giant Clams in the Sea and the Aquarium. Liquid Medium, Tampa. 227pp.

Knop, D. 1996. Giant Clams: A Comprehensive Guide to the Identification and Care of Tridacnid Clams. Dahne Verlag, Ettlingen, Germany. 255pp.

Kushmaro, A., Y. Loya, M. Fine, and E. Rosenberg. 1996. Bacterial infection and coral bleaching. Nature 380:396.

Leggat, W., B. H. Buck, A. Grice, and D. Yellowlees. 2003. The impact of bleaching on the metabolic contribution of dinoflagellate symbionts to their giant clam host. Plant, Cell and Environment 26:1951–1961.

Norton, J. H., H. C. Prior, B. Baillie, and D. Yellowlees. 1995. Atrophy of the zooxanthellal tubular system in bleached giant clams Tridacna gigas. Journal of Invertebrate Pathology 66:307–310.

Rosenberg, E. and Y. Loya. 1999. Vibrio shiloi is the etiological (causative) agent of Oculina patagonica bleaching: General implications. Reef Encounters 25:8–10.