It could be anchor worms.
If it is then Its Highly contagious, isolate fish immediately. (which you have done)
Physically remove the worms with tweezers. Treat the tank with an organophosphate such as Trichlorfon (active ingredient).
A repeat treatment maybe needed after 10 days to kill any newly hatched worms.
Some trade names of Trichlorfon
Anthon, Bovinos, Briten, Chlorophos, Ciclosom, Dipterex, Ditrifon, Dylox, Dyrex, Equino-Aid, Foschlor, Leivasom, Neguvon, Masoten, Pronto, Phoschlor, Proxol, Totalene, Trichlorophene, Trichlorophon, Trinex, Tugon and Vermicide Bayer 2349.
HTH