Seachem Paraguard Aquarium Fish Disease TreatmentParaGuard is the only fish and filter safe aldehyde based (10% by weight) parasite control product available (for parasites on fish). Unlike highly toxic and difficult to use formalin based medications, ParaGuard contains no formaldehyde or methanol and will not alter pH. ParaGuard employs a proprietary, synergistic blend of aldehydes, malachite green, and fish protective polymers that effectively and efficiently eradicates many ectoparasites (e. g.