Ok, if you have a 6+ inch bristle worm, can it harm your fish? Or is it still beneficial? Cause i saw it this morning when i checked da tank, and it was actually a little scary. Thanks.
I would imagine it is pretty unlikely but I don't think you can unilaterally trust a large bristle worm. A healthy fish should be able to avoid any attempt but a small sleeping fish could fall victim. Small fish like clown gobies and dragonets come to mind due to their sleep patters. I saw a small one of mine eat a PE mysis shrimp quite quickly. Obviously this is not a live fish but it didn't take long for this one to gobble down a good size feeder fish:
http://www.youtube.com/watch?v=t23eylcc-WE
It does depend on the species of bristleworm.
As mentioned a large enough one of some species could definitely predate sleeping fish. I would definitely worry about a 6 inch bristleworm, especially if you cannot ID the species.