Must have been something else cause Ick only gets worse not better.
That's not necessarily true. Ich looks like salt specs on the fish. On slow moving fish you need to make sure this not accumulated air bubbles from a protein skimmer or other source.
Healthy fish can fight off the parasite or even develop an immunity after repeated exposure that they survive.
Fish seem to become (more) susceptible to infestation when stressed. This could be caused by overcrowding, poor water conditions, poor diet, being harassed by other fish, etc. Removing the cause of stress for an otherwise healthy animal may be all that is needed for the fish to fight off the parasite. That could be several PWC
, removing the harassing fish or thinning the tank population, etc. This does not indicate that the tank is free of Ich, only that it is controlled.
To rid a tank of Ich you much remove all fish from the tank and treat them in a separate QT tank. The best method (IMO
) is Hyposalinity
in a QT.