This is one of three that came out to swim in the Bangerter tank last year. They are easily a foot long in this 180 gal. tank. They are still there and cause no harm. 99.9% of Bristleworms are our friends. They eat detritus and dead fish.
If you do find one that seems to be chomping on coral, they are fun to catch by placing some meaty food wrapped in a couple layers of bridal veil material tied with fishing line or thread that you hang on to out of the water. When the worms go into the folds and through the holes of the material, pull the ball out of the water by the line and dump the bristleworms