Columnaris Infection in Fish

Columnaris is an infection caused by the gram-negative, rod-shaped bacterium Flexibacter columnaris. The infection is characterized by white or gray wool-like growths on the mouth or skin of the fish. It is sometimes referred to as mouth fungus, even though it is a bacterial infection and not a fungus. However, this disease does resemble a fungus.

The most common causes of columnaris infections in fish are due to the following:

  • Injury
  • Poor water quality

This bacterial disease is often seen in the mouth region. This may be because fish are more likely to be infected after an injury of some sort and fish often damage their mouth region. For example, I once had an angelfish that damaged its mouth during courtship behavior. It develped a columnaris infection soon afterwards, which made it look like it had a small piece of cotton stuck in its mouth. Thankfully, it was successfully treated with Maracyn and lived for several more years. Fish with columnaris infections in the mouth region may be unable to eat.

Columnaris is a little more difficult to diagnose when it occurs on the body rather than the mouth. For some reason female guppies are highly susceptible to columnaris infection on their bodies. The appearance of columnaris mimics several other diseases in fish (e.g., Ichthyobodo or fungal infections). If you have a female guppy or other livebearer with smooth, whitish-gray patches on her body I would suspect columnaris. On the body it doesn't seem to form the wool-like tufts quite as much as when it occurs on the mouth, making an accurate diagnosis even more difficult. As the disease progresses the grayish-white patches do seem to become less smooth however.

Poor water quality can make your fish more susceptible to a wide variety of infections, not just columnaris. Regular partial water changes, not overfeeding or overcrowding fish, and a good filtration system should help prevent columnaris.

Treatment For Columnaris

Because columnaris is a gram-negative bacterium it seems that you should use Maracyn-Two to treat it. Maracyn-Two treats gram-negative bacterial infections and contains the antibiotic minocycline. Conversely, regular Maracyn treats gram-positive bacterial infections. Strangely Maracyn, which contains the antibiotic erythromycin, states that it is effective against columnaris. Perhaps the erythromycin in Maracyn is also effective against some gram-negative bacteria, such as columnaris.

Perhaps this is why a lot of people advocate using both Maracyn and Maracyn-Two to treat columnaris. It does seem that the Maracyn manufacturers (Mardel) should know what they are doing and so Maracyn must be effective. Unless the infection in your fish is really severe I would use Maracyn for the full 5 days and if the infection doesn't go away or at least seem to improve then I would use Maracyn-Two. For severe columnaris infections I would use Maracyn and Maracyn-Two simultaneously.

If your fish develops columnaris you could also use Melafix by API. It has antibacterial properties. However, if this doesn't clear up the problem I would do a partial water change and then use Maracyn. Keep in mind that Melafix probably isn't as strong of a treatment as Maracyn. Columnaris will be fatal if not properly treated.

Maracyn I and II No Longer Available!

Unfortunately, it seems that Maracyn I and II are no longer for sale and so you'll need another antibiotic for columnaris. I'm not sure why it isn't availble, because those products worked great for a variety of fish diseases.

At this time, I'd suggest Furan 2, which kills a variety of gram negative and positive bacteria.

The antibiotic tetracyline would most likely work, but will discolor your water. It is best to use a "hospital tank" and treat only the ill fish, but I'd try the furan 2 first.

Also, a lot of freshwater fish diseases can be helped or prevented by adding aquarium salt to the tank. Salt helps to prevent bacterial, fungal, and parasitic infections.

You can buy aquarium salt specifically made for freshwater fish, or you can buy uniodized salt at the grocery store. Don't use salt with iodine in it (iodized salt)!! It will kill your fish. I usually use about 1 tablespoon of aquarium salt for every 5 gallons of water. Keep in mind that if you have a 10 gallon tank that your rocks, heater, and other items in your tank take up some space and so you may not actually have 10 gallons of water. Also, salt doesn't evaporate and so when you replace water in your tank don't add more salt to it. You can, however, add more salt when making water changes (as long as it doesn't exceed the recommended salt level). Keep in mind that some fish don't tolerate extra salt as well as others (such as Bettas).