Well Mike some will say they are not good and some say they are all right because they will stir the sand bed and eat all the uneaten food. I myself do not like them because when I had my tank running I had a ton of them, they reproduce quickly. I had caught one it was a thick as mike pinky and about 3" but he could stretch out to 8 to 10 and I know I must have had a Monster in the tank too because I watched the tank at night one time and could watch the sand move from this one it was like something out of a SIFI movie.
The thing that pissed off the most was I lost 3 Clams to them. They would attack from the bottom and eat them from the bottom up.
Oh ya you don't want to touch them either the bristles will stick to you and it can be irritating and a true fire worm well like the name says it will burn. The ones in the photo's are just like the ones I had, a true Fireworm I believe is all red. I had seen a photo in a book and the guy had done a tare down and caught a 4 foot one and never knew it was there.
How do you get rid of them? Well that's not that easy I don't think you can ever get rid of them but may be get them and keep them under control?
1. Go easy on the tank feedings if they have lots to eat they will reproduce quickly.
2. There are certain fish and crabs that will eat them as the find them.
3. Harvest them, put a some food like a pc. of shrimp in a pc of nylon stocking or small container that can be taken from the tank with ease. You do this at night when lights are off so what you will be doing is fishing in your fish tank. Tie some fishing line to the pouch of bait you have made and lower it in to the bottom and wait with a flash light and when you see a ball off bristle worms you slowly pull them out, keep a fish net under it as you lift to catch any that fall off. This will also give you a good idea how bad of a population you have. If you have it bad it may freak you out at first because its like the floor of the tank comes alive and you see all kinds of movement in the sand and they will be coming out of the rocks hey if you got kids and want to freak them out, you know like next time your BAD your going in the tank to catch the worms HA HA HA.
Well I hope this maybe of some help to you and let us know how you make out.
All the best and good luck!
Mike