I have had no experience with columnaris but I also wondered if that might be it by your description. I gather stress and a change of water conditions can cause it and I might be wrong here, but I think it is something - like white spot - that is always present but needs a trigger. If it is columnaris your other fish in the tank will be at risk.
Try putting a post into the diseases section. You will get a bigger response by fish keepers more experienced with diseases. Just do a description of what was wrong with the fish, the sized tanks they were in, the type of filter running and the length of time you had had the fish for. You will field a lot of questions and very likely a good solution to your problem.