Other protist parasites
Other protist parasites. There are many other weakly parasitic ciliates and flagellates, both external ones, like Ichthyobodo (formerly known as Costia) and Trichodina, which are treated like Ich, and internal, of which the most notorious is the flagellate Hexamita, which is treated with Metronidazole ("Flagyl") at 250mg/5 gals, in a single dose. Seachem makes a metronidazole for the aquarium market. These others aren't true "obligate" parasites, that must find a host or perish, but merely opportunistic organisms that will take advantage of a fishes' reduced defenses. Most of the "weak" parasites will only trouble fishes in water that is too rich in organics, where they can multiply out of control.