Typically a large population of bristle worms is a sign that a tank is being overfeed. They are scavengers and eat the leftover food. Some are good, lot of them a bad IMO. If you cut back on the amount being feed, the population will reduce. I would suggest removing some of the big ones if you can because they can begin eating other things (soft corals and clams) as well. Anything over 2" I flush. The small ones make good house keepers IMO.