Addendum1-- pH
Lowering the pH/hardness may help..
There is a well known researcher of columnaris--- Decostere. published a paper in The Journal of Fish Diseases( 1999,22, 1-11, article was ... Influence of Water Quality and temperature on Adhesion of High and Low Virulence Flavobacterium Columnaris Strains to isolated Gill Arches). This paper was an excellent set of controlled experiments examining the conditions which affect the bacteria's ability to attach to substrate. This attaching is the first stage in a columnaris infection. What the study found was that most of the strains of columnaris are inhibited by salt, and require certain minerals to attach. remove the mineral and they cant attach well. This was shown for all but the most pathogenic strain. This might explain why pH worked once , but not the other time. -- just a thought.
If anybody wants the article I can email it to you
Addendum 2
symptoms are any of these... yellowish or whitish lesions on the skin, fins , Gills. Usually the area affected looks kind of pale compared to the rest of the fish. To me , it looks mildew. One of its common names is "saddle patch" because it can cover the fish dorsal and lateral, resembling a saddle shape.
Large areas of skin and mucous can be shed as the bacteria secrets a special digestive enzyme where it is attached to the fish.
Aside from the above general symptoms, The best way to ID it is to take a scrapping of the affected area and look at under a microscope and look at it as a wet mount. This bacteria is actually pills up and forms columns that move under the scope- hence the name.
This bacteria is a major problem in aquaculture and has been studied pretty extensive. A real good genera article is put out by the Southern Regional aquaculture
center, pub#479... Down load it @ http://agpublications.tamu.edu/pubs/efish/479bfs.pdf
hope this helps,
al