Got a pic or something? If it is some type of parasite you could medicate, be careful if you have inverts like snails when using such treatments.
If there are visible parasites in the tank you may as well treat the whole tank, quarantining fish at this point would be useless as they have all already been exposed.
Does this worm look like the end of it is slightly split like a snake's tongue? It could be an anchorworm, which you have to pull out yourself with tweezers. I had one when I was younger on a molly and I took it to my LFS because I didn't have the guts to do it myself and the guy just scooped up my little molly with his hands and pulled it out. After I treated for parasites and never had that problem again.
it looks like the worm has protective cover over it that made of some sort of clear mucus. i was thinking about throwing the fish away, it's just a small neon tetra.
i wouldn't get rid of that fish just yet. i'm sure there are relatively easy and non-toxic ways of dealing with parasitic worms. besides, there may be more worms or eggs in your tank, so it may not help to get rid of the infected fish.
sorry i can't recommend a treatment, though, since i've never had to deal with parasites besides ich. Knock on (drift)wood
o.k. here is update. you guys are great! i pulled the worm out. it was very hard, he was really hanging on. so far the neon is alive, but i'll give it few days and will update you. unfortunately, i torchered him for about 10 min.
you dont. that will only stress the fish even more then they are right now in your tank.
you can pull worms if no other fish are infected, and you dont have the money and/or space for a hospital tank. it probably would have been better to isolate him and mdeicate for parasites ( i think this is especially important if the fish is expensive exp. discus, zebra plecos)