Happen to be a clown loach?
Usual anti-parasitic meds are Praziquantel (found in Hikari PraziPro) and Metronidazole (Seachem MetroPlex/AquaZole).
There is Levimisole HCl (hydrochloride) and Fenbendazole, though those are considered more powerful dewormers. I haven't used those two personally so I don't know how well fish, plants and inverts tolerate it, but with stronger drugs, I usually consider them harsher on livestock as well. Again, I can't say that for a fact though as I haven't observed fish undergoing treatment with those meds. Sure land animals tolerate perfectly fine, but not so sure thats the same for fish. Could be completely wrong though.
So with that said, I usually prefer to use gentle meds as a first approach, since if a med is stronger, but harsher, it may induce enough stress on the fish to have it succumb. For this case, I would first try Metronidazole.
To give the fish the best chance, if he is still eating, try and overfeed the fish to give him as much nutrition as possible. If the fish isn't eating, then yes, treating the water column is necessary. I actually prefer to dose the water column since its more of a sure thing.
Note, a secondary round/course of treatment is often recommended to kill off baby parasites that may have hatched after initial treatment.