Columnaris (Cotton Wool Disease, Flex, Flexibacter)

What is Columnaris?

Columnaris disease is caused by Flexibacter Columnaris bacteria. It has many nick names including cotton wool disease, cotton mouth, flex or mouth fungus. It is a disease that is often mistaken for a fungal infection due to lesions that have a mold-like appearance. Infection occurs in  fish that have been stressed out by unhealthy water conditions, poor diet, or other stress inducing activity such as bullying and being carelessly transported. Columnaris can be an internal or external infection so there can be a wide variety of symptoms associated in different cases.


What are the symptoms of Columnaris?

stringy white growthsBetta Columnaris
cottony patches (white, grey, or yellowish brown)
mouth looks “moldy” and starts to decay
fins will seem to erode which could possibly be a case of fin rot occurring due to the stress of flex
ulcers or lesions (advanced stage)

if gills are infected:
change in color(gills turn brown)
gasping at surface
rapid gill movement


What to do before medicating…

Before you do anything, if your fish is in a tank with others, you need to quarantine the affected fish in a separate tank. This will help prevent the whole tank from becoming infected.

Once your ill fish has been removed from the others follow these steps on the original tank for further infection prevention:

1. Perform a 25% water change making sure to siphon the gravel of organic materials that can be a food source for the bacteria. Be sure to remove all decorations from the aquarium. You can either soak the ornaments in a hot water and bleach solution, or let them sit out until dry. The lack of hydration will kill any living organisms on your tank supplies.

2. When you add water back into the tank add aquarium salt. This will aid in healing by encouraging the production of mucus and reducing the number of pathogens. Bacteria and parasites do not have the organs necessary tolerate the change in sodium levels.

NOTE: If you have Corydoras or “Corys” in your tank refrain from using aquarium salt. They do not tolerate brackish water and could fall ill.


Medicating the affected fish…

betta fish medication

(click to enlarge)

Now to medicate. Columnaris is a negative-gram bacteria. You will need to treat both the tank and the “hospital” tank with an antibiotic. Be sure the medication you choose treats negative-gram rather than positive. A few examples of effective medications include Tetracycline and Kanacyn (Kanamycin sulfate).

Copper sulfate can also be added. It is used as an all-in-one treatment because it can cure infections caused by fungus,

betta columnaris medication copper sulfate

(click to enlarge)

bacteria, parasites and even kills algae. A word of warning, however, it is often overdosed so be absolutely careful when measuring. Also, when used to kill algae, it gets re-released into the water once the algae dies.


It can be very difficult for a betta or any fish to recover from this disease. So be sure to treat Columnaris as soon as you can diagnose it.

About Angela Soup

6 Responses to “Columnaris (Cotton Wool Disease, Flex, Flexibacter)”

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  1. liz says:

    im going to be leaveing for 9 days and i just realised that this is what my fish had …. i had three diffrent tanks two of them got infected , my ciclid and my betta … my ciclid died instantly … he contracted it from the food and died within a day of the cotteny patches showing , my betta has started showing the same thing and now cant eat without alot of struggle getting to the top of the tank , he stoped moveing this morning but is still alive , what should i do ?

  2. sean says:

    hi my japanese fighter has cotton wool disease around his gills and is allways near the surface he had this for a couple of days now im puttine the treatment the pet shop gave me into the tank and it hasent worked so far i had other fish in the same tank before i got him but they died before i got him and the treatment did not work on those fish either i changed the food and he still caught it ive even changed the gravel and all of the plants in the tank he is in the same tank as the other fish because i have no other tank to put him in just yet he is the only fish to have it ive added another filter into the tank and thats cleard most of the cotton wool from the tank i dont know what else i can do except for put what the pet shop told me to put into the tank please help thanks

    • Angelasoup says:

      What did the pet store employee recommend? I assume it was Tetracycline. Yes proceed to treat your tank and use the medication for the full time as recommended on the package you purchased. Also remember to remove your carbon filter during treatment. I would also advise you to remove the tank decorations and either soak them in a mild bleach water solution or simply let the ornaments dry out completely to be sure any pathogens are killed. Also you can add plain aquarium salt to your tank to aid in recovery. Salt will increase mucus production on your fish as well ass kill water-borne pathogens. I hope this answers all of your concerns!

  3. Worried says:

    Hi Angela, I have a betta with cotton disease… or so we think. He has a small amout of white slimy looking stuff on his fins and goes between laying on the bottom of the tank or vertically taking air from the top of the tank. This all started yesterday and first thing this morning we called our pet store where we got or betta and they recommended treating our water with Pimafix. We are hoping this works because our 2 year old is extremely attached to him. Are we doing the right treatment or should we be doing something else? We are hoping it isn’t to late for him. Please help!

    • Angelasoup says:

      Hello there, sorry to hear about your betta issues. PimaFix is a good fit for treating Columnaris. Just be sure to not use too much because betta’s are a little more sensitive to the tea tree oil. Stores also sell BettaFix which is pretty much a diluted version of PimaFix so you actually saved yourself some money. Also adding some aquarium salt will help kill water-born pathogens and aid in mucus production. Betta’s have a thin layer of mucus that coats their scales and fins to aid in healing and disease prevention. Hope he’s feeling better 🙂


  1. […] I think this is what the Betta has Columnaris (Cotton Wool Disease, Flex, Flexibacter) – Betta Info […]

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