Two things have to happen for a disease to occur: 1-a pathogen is present (which is ALWAYS true, no matter the UV
, quarantine routine, etc.) and 2-the fish are stressed. If you provide high quality food and water you will have very little stress and avoid most health issues completely. Unless you really do it right, cramming a sick fish into a smaller tank to bathe in meds will not remove the stress, it will just make things worse. Ich is easy. IME you may not have to do anything (except remove the stressor if one is known) and it will go away on its own. Otherwise just up the temp and add salt.