FWIW, the other night while looking for books on aquatic plants, I found what seems to be a good book on fish health. It includes numerous detailed (and kinda disgusting) photos of various fish diseases, parasites, etc.
I will preface the remainder of my comments by saying that I'm a newbie and have no experience with fish health problems...
The closest thing in the book to your photos is "gill maggots" (Ergasilus). These are a crustacean and what you see (if it's what it seems to be) is an egg sack. The book says you can treat the water and it will eliminate both adult and juvenile parasites. For treatment, it says to use an "organophosphorus insecticide" and names "metriphonate" (aka trichlorfon) as one of these. One or two "courses" of treatment should do the trick. It seems mature females can live "perhaps" up to a year! (The male parasite dies after mating (typical! <g>).) A "course" is 7-10 days with .25 to .4 mg / liter. Seems there should be a commercial product with this chemical in it (just follow the directions) and you should be very careful using it (seems there's a question about it having effects on people). A google search on "metriphonate" will get a bunch of info.
It warns that some fish such as "orfe, rudd and piranhas and marine invertebrates" are very sensitive to these medicines and should be removed during treatment (and could be given a 30-minute potassium permanganate bath before putting them back in the tank).
FWIW,
Liz
