Mastering Freshwater Aquarium Ecosystems
Bookmark This Page

Fish Health (Cont.)

Before treating for fungus you must first determine the cause. It is best to move the infected fish to a quarantine tank for treatment. Typically fungus grows on dead tissue that has a bacterial infection.  Fish that have a fungal infection should be treated with an antibiotic such as penicillin or ampicillin, along with methylene blue.


Columnaris (aka. Flavobacterium columnare, Flexibacteria, Fin Rot, Tail Rot, Mouth Rot)

Columnaris causes a white patch on the fish often resembling a saddle. Columnaris is a more solid white in the middle and fades outward. Columnaris is a gram negative bacteria, and when viewed under a microscope looks like rods. Fish that are exposed to high ammonia levels or low oxygen levels are at greatest risk of contracting columnaris. It is most commonly seen on livebearers that have been newly acquired. Columnaris will normally appear on fish within the first week of acquisition. It needs to be treated as soon as it is noticed. Mortality can occur in fish that contract columnaris within three days. Columnaris can quickly spread in warm water. Columnaris is a freshwater fish disease that cannot tolerate salt.

Koi with Columnaris

Koi with a classic sign of a Columnaris infection. Photo by Dr. Thomas B. Waltzek.

Columnaris can be treated with oxytetracycline, neomycin, kanamycin, and salt (sodium chloride). Hydrogen peroxide can be applied to areas of infection with a cotton swab to kill bacteria. Hydrogen peroxide can be purchased off the shelf at your local pharmacy or grocery store.

If possible lower the water temperature to 75°F (24°C) to slow the reproduction rate of the columnaris bacteria. Salt should be the first choice for treating columnaris provided the fish you are treating are salt tolerant. Salt concentration should be at least 5 parts per thousand (ppt) but preferably 10 ppt would be best. This equates to about 1 to 2 tablespoons of salt per gallon. You should use a marine aquarium hydrometer to check the salt concentration of the water. Fish should be treated for at least 10 days, then the water can be returned to normal freshwater through water changes. Fish should be closely observed for three weeks post treatment to make sure there is no recurrence of columnaris.

Environmental Conditions

Some symptoms and conditions fish can display can be directly attributed to water chemistry. Often symptoms expressed by fish are misdiagnosed as being caused by a disease.

Aquaworld Sponsor