Parasite worm in fish

Question
I have an aquarium with about 20 small fish. Yesterday I found that my silver hatchetfish had a small round orange thing that was moving (kind of a worm) hanging from the anal orifice.

I became really concerned, the water is in good condition and the only new thing in the tank is a couple of plants (which I cleaned and disinfected thoroughly) that I added two days before I found about this. My two questions are if I can do something to help the fish? and if this can be contagious?

Thank you very much.  

Answer
Hi Fernando,
Thank you for your letter. Heck of a hobby, ain't it? You think you've seen everything . . . and then something like this.
I would say that it was some sort of parasite that was in the fish when you bought it. Hopefully you were able to remove it and throw it away. I would suggest that you just forget it. If it happens again, then be concerned. It could have been in the fish all along, could have come from the plants you added, could have been something that blew in or fell in the tank. Who knows? It could be contagious, but it could also be a one time thing.
Don't go balistic with chemical treatments.
The best prevention is to keep the water healthy by limiting food and using the product "Cycle" to treat the water. Cycle is not a chemical, but friendly bacteria. Fish need bacteria to digest, to keep their silky coat healthy, etc.
Here's a full explanation:
http://steamboats.com/aquarium.html
If it comes back or if you want to discuss further, please write back.
Nori