I recently received two letters, each one describing a different pest or perceived pest (in this case, dinoflagellates and flatworms — planaria). Flatworms come in many shapes and sizes. The ones we normally consider pests tend to be small (less than 5 millimeters in length), flattened worm-like creatures. They can be brown, green or red in color, often with a white stripe down the middle of the back. These animals feed on bacteria and coral slime, and are thought to be relatively harmless.
The problem comes when there is a population explosion in the aquarium and they begin to cover every available surface, including your corals and other sessile invertebrates. They become a real eyesore and can also severely cut back on the amount of light the corals receive. At this point you have three options: leave them alone and wait until they begin to disappear — which often happens, siphon them off the corals every day until their numbers begin to decrease, or find a biological control.
Biological controls are tricky because there is no guarantee that they will work, and if they do work, what will they eat once the flatworm population declines? Saltwater fish are the most effective biological controls for flatworms. Members of the genus Valenciennea (sleeper gobies) have been observed to reduce populations of flatworms effectively, but they tend to waste away in aquariums once the flatworms decrease in number.
Wrasses are very interesting saltwater fish in that if there is a life-form that occurs on a reef, there is usually a wrasse that eats it. Both Macropharyngodon (leopard wrasses) and Anampses wrasses are suspected to eat flatworms in aquariums. In the few cases in which I have experienced flatworm outbreaks in a reef system, the populations eventually disappeared in about six to eight weeks.
Flatworms often appear in new aquariums containing live rock, when new pieces of live rock are added to an already established reef aquarium, and when new pieces of coral are added. This is why it is prudent to carefully inspect new arrivals for suspicious hitchhikers, and to quarantine new pieces in a separate saltwater aquarium to make sure they are pest free. Often, vigorously shaking the piece in a bucket of seawater is enough to dislodge flatworms. A more drastic procedure is to dip the live rock briefly in freshwater for 30 to 60 seconds, causing them to drop off.
Dinoflagellates can become a real pest in saltwater aquariums, and have caused more than one hobbyist to completely tear down and sterilize his or her aquarium. This tiny, single-celled protozoan can multiply into large mats of brown, filamentous growths, often with air bubbles attached to rising filaments. When examined under the microscope they look like very tiny spheres embedded in a gelatinous goo!
There is no sure-fire way to rid a saltwater aquarium of this plague, and often it is merely the shock of a change that causes them to disappear. Treatments have included raising the pH to 8.5 for several weeks, dropping the specific gravity to 1.016 rapidly and keeping it there for a few weeks, raising or lowering the temperature, using phosphate-lowering compounds, and turning off the lights for a few days to a week. Sometimes a combination of the above will cause a positive result. Unfortunately, there does not seem to be any organisms that will eat these beasties either!
As you can probably tell, many of the above options carry their own dangers for the other inhabitants of the aquarium, but drastic problems sometimes require drastic efforts. You may lose all your animals or you may be successful. The alternative, however, is to do nothing, and that will eventually lead to a very nice display of dinoflagellates on live rock and little else.