Please read the article Hholly posted, the original is "An Aquarist's Worst Nightmare".... Do read it.
Anyways, Camallanus will shrink into the fish if treatment that is beneign to it begins taking effect.
It efficiently stows itself away, possibly for months. By that time, it would already have built up immunity and been up and ready for another dose.
I never buy fish from big brand stores, just decor and gravel and such. I have bought 6 fish from Petco. A betta, 2 goldfish, and three platys.
The only survivor is one of the platys.
Needless to say, it goes to show how some petsmarts and petcos, while some are extremely well maintained and full of knowledgable employees, simply fall into a state of denial and continue to sell misinformation and sick fish to customers.
I wonder. How do they sleep at night?