Fungus is a common fish disease that can prove fatal if not treated early. Although there are several water moulds that can affect fish, the most common and significant water mould is Saprolegnia, a filamentous fungi. In common with all moulds, it feeds by secreting digestive enzymes onto the surrounding area. These enzymes break down cells and tissues enabling the fungi to absorb nutrients such as proteins and carbohydrates. Fungal infections are called mycoses .
Water moulds consist of a tangled mass of individual filaments called hyphae.
As the mass grows it forms compact tufts that are collectively called a mycelium . The mass of fungi filaments or mycelium can be seen clearly without a microscope. Moulds reproduce by releasing thousands of spores into the surrounding water. The fungal spores are highly resistant to drying and chemical attack and thus Saprolegnia spores and molds are common in all ponds and tanks. It is believed that the fish mucus contains effective fungicides that prevent fungal growths under normal circumstances.