My black molly has developed a cottony looking growth around her mouth and body, and her tail is looking frayed. I suspect Columnaris but cant tell for sure, I separated her from the rest of the aquarium into a temporary fish bowl (with frequent water changes because there is no filter!) but I need to get her better situated today.

What is the best course of treatment? Do I need to treat just her or the whole tank? Once treatment starts can she go back in the main tank or do I need to buy a smaller "emergency" tank??