neon tetras are very thin

1 of the 11-13 neon tetras in my 90 gallon planted tank has been very thin since I bought it (~5 months ago) and recently I notice that 2-3 others are now thin as well. The cherry barbs and other neon tetras are very fat and healthy so I have no idea what could be causing this.

I was thinking the one neon tetra just wasn't eating well but now that more of them are thin I am afraid it might be a parasite. I can't tell anything wrong with their swimming- they are just as spazmatic and lively as all the other fish when I go near the tank.

Any ideas would be appreciated

I did look for that but could not see any. They get really excited and move a lot when I get near the tank since they think I am going to feed them and I have read that the worms hide if the fish moves a lot.
Are worms normally very obvious and easy to see?

Last night I checked on the tank and did see something hanging out of the bottom of a skinny neon tetra but could not get a good look. I am guessing that was a worm. What do I need to treat them with? Should I catch all the neon tetras that look infected, all the neon tetras including healthy ones, or do I need to treat the entire tank?

What kind of worms are they and how should I treat them? two more neon tetras look thin and the original sick tetras are getting much worse