Fin rot and columnaris
Bacteria that attack the fins, mouth and skin, nearly always after poor water. When water changes are enough, and when more is needed.
Bacteria waiting for a chance
The bacteria behind fin rot and columnaris live in almost every aquarium, even healthy ones. A healthy fish has slime and skin that keep them out. It is when the fish is weakened that they get a hold.
What most often weakens the fish is the same thing: poor water with ammonia, nitrite or a lot of nitrate, too many fish, too few water changes, or stress from fin nippers, a move or the wrong temperature. The Norwegian Food Safety Authority (Mattilsynet) writes that clamped fins, damage to fins and body, and fungal infections are signs that the fish are not thriving or that the water quality is poor.
That makes the disease a warning, too. The fish with bad fins is often just the first to show that something is wrong with the aquarium.
Fin rot
Fin rot starts at the edge of the fins and eats its way inward.
- Early: the edge turns uneven, ragged or milky white, sometimes with a reddish streak.
- Later: the fins get shorter, develop holes and can rot all the way to the body. The fish often keeps them clamped.
Be sure to tell fin rot apart from fin nipping. Bite damage shows up as uneven "notches" that appear suddenly, often on long-finned fish, and the edge is clean. If you see the fins healing between attacks, a tankmate is the problem. See Fish that get along. But bite damage can also become infected and turn into fin rot.
Columnaris
Columnaris, named after the bacterium and sometimes called cotton wool disease, looks like a fungus and is often mistaken for one. The fish gets a grayish-white, fuzzy or threadlike coating, often:
- around the mouth, which can look like a white lip
- on the back, where a pale patch can spread like a saddle
- on fins and gills
What sets columnaris apart from ordinary fin rot is the speed. It can move fast, from a small spot to a dying fish in a few days, especially in warm water. It also spreads more easily between fish. True fungus more often grows in wounds or on dead eggs, and looks more like fine, white cotton tufts.
How to treat it
Always start with the water, whatever it is.
- Test the water. Ammonia and nitrite should be 0, and nitrate low. See How to test your water.
- Change the water, about a third, and do it again over the next few days. Mattilsynet writes that it often helps to change some of the water and check that the temperature is right.
- Remove the cause: feed less, thin out the fish if the aquarium is overstocked, and move fin nippers.
- Isolate the fish if you can, in a small tank with water from the aquarium. That shields it from tankmates, and you can treat it without dosing everyone.
Mild fin rot can clear up on its own once the water is good. The fins can grow back over a few weeks, often with a clear or white edge where the new tissue is.
Columnaris, and fin rot that keeps getting worse, need more. A fish store can advise you on antibacterial treatments for fish. Follow the instructions, and take activated carbon out of the filter during treatment. In many countries antibiotics for fish need a prescription, so check your local rules and talk to a vet. Many people also lower the temperature a couple of degrees for columnaris, since the bacterium grows faster in warm water, but only within what the species can handle.
How to avoid it
- Change some of the water every week, and keep ammonia and nitrite at 0. See Water changes: how much and how often.
- Do not overstock the aquarium. See How many fish can I keep?
- Choose fish that get along, so long-finned fish are not nipped at.
- Quarantine new fish. Columnaris can come with fish from the store. See Quarantine for new fish.
If you are unsure what is wrong with the fish, start with Sick fish: how to spot it and what to do first.