Focus is a polymer for internal infections of fish. It may be used alone or mixed with other medications to make them palatable to fish and greatly reduce the loss of medication to the water through diffusion. For use in freshwater or marine aquariums. Focus is recommended for the treatment of certain issues in marine and freshwater fish only. It IS NOT recommended for treating any style aquarium containing invertebrates, corals, crustaceans, or elasmobranchs (sharks and rays).