External parasites can also irritate the skin of fish, causing damage that may lead to secondary bacterial and fungal infections of the body and tail. Before using any pond medication, be sure to remove any activated carbon from your filter system, as the carbon will filter the medication out of the water. In certain instances, pond fish that are very heavily infested with parasites may not be able to recover despite treatment. Damage to the gills, secondary bacterial or fungal infections, and stress to the immune system prevent recovery. The first dose is half strength to reduce chances of fish loss due to treatment stress.
Early treatment is essential for success.