Goldfish and koi are often called pondfish, because they are two popular breeds often bought together to inhabit a resident's backyard or decorative house pond. As living creatures, however, they are susceptible to diseases, which may infest the pond if it is not properly cleaned or aerated. A common disease that results in the formation of wart-like sores on the fish is known as carp pox or lymphocystis.
Carp pox results from an infection of the herpes virus. If a fish catches carp pox, it is not usually fatal, as the warts and virus themselves are not lethal. The placement of the warts that develop from the virus, however, can cause discomfort to the fish if they develop on a necessary area such as the fins or gills. The virus that causes carp pox is easily transferred between fish, so overcrowding will cause it to spread to other fish in your pond.
This virus causes warts to form on the outer scales and gills of pond fish. These warts look like small balls of wax. After a few weeks, the warts should decrease in size if the fish is kept from enduring too much environmental stress such as sudden fluctuations in pond water pH, salinity or metals.
The herpes virus unfortunately remains with the fish for the rest of its life once it is caught. Therefore, peeling the warts off the fish's scales is not a long-term solution to curing carp pox. Instead, the environment in which the fish lives must be treated to remove the virus. If the water is not already being filtered, run the entirety of the pond water through a filtration system to remove any physical clumps of food or debris on which the virus may live and feed. It also may be necessary to disinfect the water using a UV filtration system or chlorine shock.
To prevent the spread of carp pox, do not overpopulate your pond with fish. Make sure they have enough room to swim free of contact with other fish. Keep the pH of your pond regulated around 7. Sudden fluctuations in pH can cause stress to the fish which will weaken its immune system and make it more susceptible to contracting the virus. The main source of the herpes virus to pond fish you already own is the introduction of an already diseased fish into the new environment. Upon purchase of your new goldfish or koi, ask the previous owner if any symptoms of carp pox have ever been observed.