Keeping goldfish or koi in your pond can be a satisfying endeavour; your fish will become your friends and can live for many years or decades with proper care even in Northern outdoor ponds in colder US winters. There are some common goldfish and koi diseases and illnesses that can be treated with medications. It is always distresssing to find our fish with ulcers or discolorations or growths on their extremeties but with care and proper medication and water treatment many of these conditions can be reversed and are often non fatal.
Some of the most common problems with goldfish are: Fin rot is a literal rotting of the fins. It is not fatal when treated. Ick, or White spot disease, appears as white spots like sugar grains in goldfish and can be fatal but is treatable. It can be brought into ponds or tanks by contaminated equipment or new fish who carry the Ichthyopthirius. Medication and water changes as well as cleaning and quarantine are required. Other common issues that can be treated with our popular goldfish and koi medications are bacterial dropsy, fungus, milky skin, fin/tail rot, bulging eyes, mouth rot, raised scales.
One problematic issue with ponds that cause fish illness and death is ammonia poisoning which is not specifically treatable by medication but can be eliminated by pond water testing and proper filtration and aeration or by emergency ammonia reduction additives.