Most of them are good, they make great scavengers, although its quite possible for their populations to get out of control, especially if the tank is overfed, or there is a build up of detritus which is common when using curshed coral for the substrate. Not to say this is a bad thing, as mentioned many people pay good $ for BW, but a large population of BWs can be very unsightly. Many organims will feed on them including arrow crabs, coral banded shrimp, pseudochromis as well as many wrasses. In the case of the shrimp and crab they will actively hunt down bristles, but in time may also hunt down small slow moving or dormant fish. As mentioned the Pseudochromis can be very agressive and most of the wrasses that will put a dent in a large BW population get fairly large, and will also cosume other inverts such as hermit crabs. A 6 line would be ideal, but its size limits the number and size of the worms it can eat. ALthough if you had a 6 line that ate BWs it would help to keep the population down as it would be taking out the younger worms before they have a chance to reproduce. The best route would be to limit their food supply, and manually remove them when possible via a trap, or if possible siphon them out during water changes. HTH