Most people say that they are a benefit for your tank. I had some zoas that were slowly disappearing. I seen a huge bristle worm about a foot long and it's girth was the size as a pencil on top of the zoas. I got one of these bristle worm traps
http://www.aquacave.com/bristleworm-...alife-932.html. Looks like a cheezie gimmick but works great. I watched the worm try and get in and could'nt fit but it also got stuck and I was able to get the worm out. When I put the trap in the sink to remove it from the trap it shrank itself to about an inch long and really fat. Since than my zoas are no longer disappearing. They may be good for our tanks but my zoas are too expensive to be feeding to these ugly worms.