The pics are too dark to know which exact parasite it is but it's most likely either fish lice or gill flukes. Both can be treated with Fritz Paracleanse (or Praziquantel alone) or Dylox from API. You can also try doing Saltwater baths for the fish to help burst the parasites osmotically but still treat the whole tank as any fluke or lice eggs that were laid in the tank will need to be treated as well as the ones on the fish. You will probably also need to treat the fish with an antibiotic or similar med after the bugs are gone as they are damaging the gills of the fish so they will be leaving some wounds behind as they fall off.