The only thing I want to add is that when fish are very sick and weak
and consequently crashing on the bottom, they can have difficulty with
the current generated by an airstone on the bottom of a small
container. One can create a weaker current by mounting the airstone
higher up on the wall of the container. The same thing can be
accomplished by bleeding off some of the air, but that also reduces
surface agitation. Basically one just has to adjust things based on
how it looks.

The environment has to be as stress free & pristine as possible to
heal. Currents in small containers can be stressful. I find
sometimes also reducing room light is calming.

Just another $0.02