I know i've seen something that looks like these somewhere on the web but I cannot find the site now and I forget if these are good or harmful:




As you can see I have quite the collection of this one type of bristleworm. At first I thought it was attacking my Cerith but I soon realized that it was eating all the hair algae off the cerith's shell...

so are these beneficial?

they are an excellent addition to your clean up crew!

now hopefully they wont grow huge and start eating my livestock

bristleworms are awesome

Hi

I noticed a year ago i had a bristleworm in my nano tank.. Being new to having a nano tank setup i left it alone.
BiGGGGGGGGGG mistake..... just recently i had to restart the tank. Get rid of it now... they are a plague. One turned into 100.. Killed a couple of fish and looks horrible in tank when feeding time. buy the way dont touch them without latex gloves. They will cause you pain..

Never heard of bristles killing fish before, perhaps the fish died and the bristles were trying to clean it up.. I have them in my tank, I rarely see them. Maybe you were overfeeding thus providing enough for them to get out of control? I like them, as many on N-R do, they do a wonderful service in the tank.

+1. If your bristle worms are fire worms, like what the OP's pictures show, then I'm 99% sure those worms could not have killed healthy fish. Fire worms have no jaws, and they can only kill things by licking them to death, which is why the most dangerous fire worm species is a coral eater.

We need more information (i.e. complete livestock list including the dead fish, water parameters, how and why you "restarted" the tank) to say that your observation is accurate. It should have been obvious over the year you had the worms whether fish were dying or not. Bristle worms are often the scapegoats of other problems...

I have a worm a lot like your pics, where the first inch or 2 of it is pink and the rest is purple. Bright colors too. I named mine Sam.