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. We are not veterinarians nor scientists, and the following information is based on our experience with raising koi. For in-depth information on fish diseases, we advise to contact an expert in the field.
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.
It is important to remember that bacteria are separated into gram-positives and gram-negative bacteria and that gram-positive medication will not have any impact on a gram-negative infection. Viruses will not be cured by medications for bacteria as well, but they may help secondary infections.
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 typically 1 pound per 100 gallon for 0.1% to reach 0.3-0.5% concentration. It is recommended to only add 0.1% per day to prevent shocking the fish. Salt is perhaps one of the most cost effective methods of treating freshwater fish, especially if the species can tolerate higher concentrations. It is particularly well at treating certain parasites such a freshwater Ich. The effectiveness of the salt will be largely based on what exactly the fish is afflicted with.
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.
A virus that is more commonly encountered is the Koi Sleepy Disease or Carp Edema Virus and can be easily mistaken for KHV. The common symptoms of this virus is when koi suddenly lay on the bottom as though they are sleeping and will typically get back up and swim normally when disturbed. These fish act normal and will even eat regularly despite the seemingly dead or dying condition prior to being disturbed. Often times, many fish are already immune to the virus after being exposed at some point in their life, however, fish bred in isolated regions are susceptible to the virus if exposed to carriers. This virus, unlike KHV is treatable, but similarly devastating if left untreated, though it is important to diagnose the disease correctly, and the fish may need to be tested by a lab. The common treatment is the use of salt and possibly temperature. Increasing the concentration of salt to 0.5-0.6% and raising the temperature up to 80°F is commonly used to prevent the virus from doing major damage to the koi as well as preventing any sort of relapse. In particularly bad cases, the salt increase may have to be done within a 1-3 day period as the deterioration can occur quite quickly overnight. In such cases, slowly administer the salt every couple of hours until reaching the desired concentration. If medicated food is available to you, this may help with secondary infections from bacteria and other pathogens that will target your fish in a weakened state. After recovering from the virus, the fish should be immune to the virus for the foreseeable future.