3 Pack of Seachem ParaGuard 500ml Bottles. ParaGuard is the only fish and filter safe aldehyde based (10% by weight) parasite control product available. 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 on fish (e.g. ich, etc.) and external fungal/bacterial/viral lesions (e.g., fin rot). It is particularly useful in hospital and receiving tanks for new fish and whenever new fish are introduced to a community tank. For use in freshwater or marine. ParaGuard is recommended for the treatment of certain diseases in marine and freshwater fish only. It IS NOT recommended for treating any style aquarium containing invertebrates, corals, crustaceans, or elasmobranchs (sharks and rays).