If you think yor fish have Columnaris they almost certainly do not. IF a postitive identification is made by successfully culturing the bacteria then and only then should an antibiotic be administered in a course of treatment of the correct duration so as to prevent the creation of an antibiotic resitant mutation of the bacteria. Much safer is to use a non-antibiotic bactericide, many of which kill both bacteria and protozoans which are almost always the cause of what many aquarists think is a Columnaris infection.

Here's what Dieter Untergasser says about it in _Handbook of Fish Diseases_ (TFH press, ISBN 0-86622-703-2)

There are two forms of the disease. During the chronic course the white areas gradually enlarge and the fish die many days later. In the acute form, the white spots spread out visbly within several hours. A population of 100 fish may all die within 3 days sotreatment must be instituted quickly. Higher water temperatures accelerate the disease (emphasis mine)

Treatment is possible but is only successfull over a long period when optimum maintenance have been established. Various methods have been used for the chronic and acute forms. The chronic form can be treated with:

1) Acriflavine: 1g to 1 L stock solution, then 1 ml/L to clean the tank and 5ml/L in a bare tank to treat the fish.

2) Chloramphenicol 40 mg per litre for 10-20 hours. Dissolve in in a small quantity of ethyl alcohol. The water will go turbid, use stong filtration and change the whole mess if it does.