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The ability to effectively treat parasitic infestations of fish is of high importance for fish culture facilities. However, tools or approved therapies for treating infestations on fish are limited. This paper summarizes results from four separate clinical field studies that evaluated the efficacy of hydrogen peroxide (H2O2; 35% PEROX‐AID) for reducing Gyrodactylusspp. infestation density. Three species of Gyrodactyluswere studied (G. salmonis, hosts: Brook Trout Salvelinus fontinalisand Lake Trout S. namaycush; G. freemani, host: Yellow Perch Perca flavescens; G. hoffmani, host: Fathead Minnow Pimephales promelas) before and after the application of immersion H2O2therapy. Parasite density was significantly reduced for each parasite × host combination to which H2O2therapy was applied. Two clinical field studies in salmonids were found to demonstrate substantial effectiveness that enabled 35% PEROX‐AID approval. Further assessments of Gyrodactylusspp. could expand the use of H2O2for controlling these parasites in aquaculture. Specifically, H2O2was effective at all levels tested (50 or 75 mg H2O2/L for 60 min for the Yellow Perch and Fathead Minnow clinical field studies; 100 or 150 mg H2O2/L for 30 min regardless of salt pre‐treatment for the Brook Trout study; and 100 mg H2O2/L for 30 min or 50 mg H2O2/L for 60 min for the Lake Trout study). Impact statementThe external parasite species of Gyrodactyluscan be very problematic to fish raised in hatchery settings. Having approved and effective treatments to reduce parasite infestations can be helpful to hatcheries. The work summarized in this paper explored the use of hydrogen peroxide as a treatment option for hatcheries to use for reducing Gyrodactylusinfestation density on four different species of fish. Hydrogen peroxide was effective at reducing Gyrodactylusinfestation densities at all levels tested. The external parasite species of Gyrodactyluscan be very problematic to fish raised in hatchery settings. Having approved and effective treatments to reduce parasite infestations can be helpful to hatcheries. The work summarized in this paper explored the use of hydrogen peroxide as a treatment option for hatcheries to use for reducing Gyrodactylusinfestation density on four different species of fish. Hydrogen peroxide was effective at reducing Gyrodactylusinfestation densities at all levels tested.