What you refer to here is a small parasitic fish called a Candiru (familyTrichomycteridae)from south America.sometimes called a willy fish,the one youve heard about that can swim up your willy when urinating in their habitat....ouch,painful!!!
Where its possible i would have thought it unlikely that this is the culprit,i would go with the lice or at least some sort of crustacean type parasite.
Here in the Uk you can use a treatment from Waterlife research called Parazin P,its a pond remedy but i have used it on all delicate fish including rays,its a one shot treatment that doesnt damage the biological filter,fantastic stuff,one tablet treats 25gals of water,i believe theres another treatment widely used called Dimilin,i have no experience with this but have heard it works.
Hope this helps you.