I was in one of my not so favorite local fish stores a while back and I overheard a customer speaking with a sales representative about his current bristleworm problem. He told the rep that he had literally hundreds, if not thousands of tiny bristleworms throughout his crushed coral substrate. He was in a near state of panic!
The sales rep immediately replied, telling the distraught hobbyist to buy special fish that would eat the worms as well as placing several small traps throughout the system (naturally, they sold them). He even went as far as to direct the hobbyist to consider replacing his substrate.
My first thought was of how unfortunate this aquarists situation was. Not that he had the bristleworms, but that he had been so misinformed about his so called "problem". So after giving it a little thought, I decided to challenge the sales rep's advice with a little of my own. As I expected, the hobbyist purchased the traps and a few new fish to deal with the outbreak regardless of my advice. If only he had known just how beneficial most marine worms are, perhaps he would have reconsidered.
The bristleworms we are discussing are actually polychaetes and cover a wide range of different species ranging from the much welcomed Christmas-tree worm (Spirobranchus giganteus), to the oftentimes dread sand dwelling varieties that often unjustly earn the title of "pest" just by there appearance.
More commonly than not, the various polychaetes we see in our systems are harmless and actually very beneficial in our systems. These worms are very prolific and eat decaying organic matter before it has a change to decompose and enter our water column. Practically every piece of live rock and every hand-ful of live sand contains some form of polychaete. You may notice them throughout your sand bed, in and around your live rock and even swimming in the water itself! Every so often, you may even encounter a species that is attached to or attacking one of your prized corals. These very few species are the only species that should cause us any concern, and for the most part are extremely rare.
A good rule of thumb when considering when or not to take action when spotting a polychaete that you are unable to identify is to ask yourself a question. Is it causing any damage? Is it attached to any of my corals? If the answer is no, then more than likely, these a re the "good guys"!
Oftentimes, sand-dwelling species may reach higher densities though this should not cause any alarm. More than likely, they have entered a reproductive stage that will soon pass, resulting in lower populations as they die back or are consumed by other organisms.
So what happens when a parasitic worm is spotted? There are many methods for dealing with the removal of these species, ranging from siphoning them out with a water hose or airline, to freshwater dips for boring species that have attached themselves to your corals. For the most part, they can just be brushed off however.
Another possible remedy for controlling bristworms are purchasing various small shrimp or maybe an arrow crab. Keep in mind that they will eat both the good and variety polychaetes. (including your ornamental varieties) Also keep in mind that arrow crabs can get very large and be a problem later down the road.
Another important point to mention when discussing the removal of many of the larger varieties, is that some species are able to inflict a powerful and irritating sting. One particular group are the fireworms..
In general, I find that these treatments and removal processes are more often than not, unnecessary and a case of mis-identification.