Treating Sick Fish

Bacteria is nigh-omnipresent, parasites can lay dormant, and there is no escaping a viral plague. It is important for aquarist and fish keepers to prevent diseases from ever coming out, not wait to treat them when they show up. For a list of potential disease, click here.
When it comes to treating sick koi, use medication that is available to you locally or easily obtained; if your medication runs out halfway into treatment, it may be potentially dangerous or counterproductive to substitute it with a completely different medication. For large ponds, it is recommended that you setup a quarantine tank to treat sick fish as it can cut the cost on the amount of medication used. Most medications contain chemicals such as methylene blue, malachite green, formaldehyde, erythromycin, and so on. Read the fine print and instructions to see what you can and cannot do; some medicines for example may react poorly with salt and poison your koi.
Aquarium salt or rock/table salt (Never use marine salt) is also a commonly used treatment for koi; avoid salt containing anti-caking agents used to prevent salt from sticking together. The dosage of salt is usually one teaspoon of salt per gallon reaching 0.1-0.3% concentration. It is recommended to only add 0.1% per day to prevent shocking the fish. Salt is perhaps one of the most effective methods of treating freshwater fish, especially if the species can tolerate higher concentrations.
Regarding viruses, there are myths surrounding the infamous KHV (Koi Herpes Virus). Koi imported from Japan when carrying the virus may not show symptoms immediately; this is largely to do KHV becoming inactive when kept in salt and higher temperatures over 80°F, a popular form of preventative. However, this does not mean that the KHV is “dead”, viruses cannot be killed in the traditional sense but rather the immune system fends off against the infection and builds resistance to the disease; the virus will remain in the "survivor" koi for the remainder of its life. KHV “immune” koi will become carriers potentially infecting healthy koi if the virus leaks out of the carrier. While it may seem cruel and unwarranted, koi infected with KHV should be humanely destroyed (killed) and the pond or tank fully sterilized to prevent further spread of the disease.