3 pack. PolyGuard is the single most effective medication to keep freshwater fish disease-free. It is an effective treatment and safeguard against all prevalent bacterial, fungal, and parasitic diseases of exotic fish. For use in freshwater or marine aquaria. When our pets are sick, we immediately want to reach for the most powerful medication on the shelf. However, sometimes the right choice to prevent stress is to opt for a gentler medication at first, especially with less severe infections. PolyGuard is among the strongest medications offered by Seachem, and is best used when you are unsure what disease you are treating or when the fish is suffering from multiple infections.