What are your parameters? What size tank do you have?
This was the puffer that you posted with the white areas, right? Did you do anything to treat that? If one fish has something, then others are exposed. There is not much that cures itself, so with no help, whatever is going on will continue.
Having a QT is wise. A simple glass 10-20g with a few pieces of PVC
and a filter or power head and maybe a heater does not cost much when compared to the potential casualties.