I agree with this wholeheartedly. Taking a fish that is already at a low ebb thanks to some disease or infection, and sequestering it in a tiny quantity of water with no filtration, no temperature control and wildly unstable water quality has never made any sense to me....I wouldn’t treat fish individually in a bucket as water parameters can change too rapidly.
If one wishes to treat and cure such a fish, it needs the best conditions that can be provided (i.e. a properly equipped hospital/quarantine tank) rather than the worst.