I believe the fish has passed, as far as treating the tank columnaris is very contagious and spreads very fast, lower tank temp to 72-76 watch fish closely, look for red spots on other fish, test water paramiters, watch for white spots on other fish, if your fish passed that fast from it then it's the fast mortality strain as there are 4 strains of columnaris, you need a gram negative antibiotic to treat columnaris, I can't say it is columnaris but I did have it once but the whole fish had white fuzz not just the mouth.