Columnaris is nasty stuff. Don't feel badly. There are no guarantees. I am always at a loss when the bad times come--and they do come. Sometimes we win one, but I lose as many to the meds as I win with them.
I agree; give that tank a long time to settle. New fish are stressed and can get sick easily and get something started.
Thank you! Yeah, I feel bad saying this but I had 2 others (that I bought already sick) die this week and while it was sad, I was like "eh, poor thing" and moved on..but I miss this little guy very much (never knew I would miss a little fish), but I'm actually also relieved he's gone, and glad he's not hurting anymore.