Seachem MetroPlex Seachem MetroPlex is metronidazole — primarily an antiparasitic aimed at internal protozoan parasites rather than a general bacterial antibiotic. It's the go-to for the internal flagellates behind conditions like Hexamita and "hole-in-the-head" disease, as well as Uronema and Brooklynella, and it carries useful activity against certain anaerobic bacteria too. If a fish is wasting away, passing white stringy feces, or developing head/lateral-line erosion, MetroPlex is the classic treatment. Good to Know: Because it targets parasites rather than bacteria, MetroPlex tackles a different problem than the antibiotic -Plex products like KanaPlex — and the two are sometimes used together (they don't interact) when a fish shows both parasitic and bacterial symptoms. It's most effective fed in food via Seachem Focus, since these parasites live in the gut. MetroPlex is generally reef-safe at standard doses. Internal parasites are easy to miss until a fish is clearly declining, so many keepers keep MetroPlex on hand as part of a quarantine protocol for new arrivals. Dosing Guidelines: Dose per the label — typically every other day for about three doses in water, or bound to food for 5–7 days for internal parasites. Remove chemical filtration while treating, and complete the full course.