An antifungal agent for the prevention and treatment of the fungal diseases caused by the water mould Saprolegnia parasitica includes the natural product sweet flag extract or its essential oil as an active ingredient and, therefore, is eco-friendly and not harmful to fish. In addition to offering effective prevention and inhibition of mould growth, the antifungal agent can replace malachite green, a known carcinogen, in the aquaculture industry.