It will kill fish by adulterating their gill tissue. The fungus compromises vascular supply to gill tissue and the affected areas die and slough out leaving gaping sockets in the gill arches. The fungus relies on high organic loads, high Nitrate levels, crowding, and HEAT to activate, and kill fish. When you examine the fish, you will notice the gills are eroded and there are necrotic sections in the gill tissues, grossly. Special stains will show starlike fungal elements, but this is not critical because treatment is simple.
Cool water carries more oxygen and eases the metabolic furnace of the fish.
The pond should be cleaned, organics in the filter and in the biofilm should be scrubbed or removed and the water should be cooled, which can and will be accomplished in part by the frequent water changes mandated by the Formalin treatment. Fish will be noticeably perkier and feel better the day after the water is cooled and the Formalin is applied. No Formalin treatment should last longer than 2 hours. Fish diseases and their control. An occurrence of a Branchiomyces sanguinis infection is described in fingerling striped bass reared at a fish hatchery in Arkansas.