Well, it might not be a parasitic problem that your fish has.
Is your fish showing any other symptoms, apart from looking thin? Is the belly sunken in? Is it eating normally? Swimming ok (not off-balance)? Any fin rot appearing or bulging eyes? Rapid breathing/gill movement? Anything else that doesn't seem normal to you?
There are lots of things that cause a fish to become thin - yes, it could be internal parasites, or a fungal infection, or bacterial (or a combination of any of those). But unless you can see any other obvious symptoms that might help pinpoint the cause, any suggested treatment will be based on a guess.
Also, the longer the fish is left without treatment the less chance it has of survival, unfortunately, and also the higher the risk of cross-contamination to other fish in your tank.
Regards - Athena