If you ask 6 people a question you'll probably get 6 different opinions on how to do something...if you ask 10, you'll STILL get 6 opinions and 4 to agree/argue..
So I'll be #7...This is what works for me...
I treat a quarantine tank and a hospital tank differently, because they serve different purposes.
If I am "planning" a new addition/fish purchase...I will set the quarantine tank up (20gal glass) a few days in advance...using an Emperor 400 HOB filter with main tank water and a cup of substrate (with bacteria) in the "baskets". This gives the tank time to stabilize and impregnate the bio wheels with bacteria...
It also gives me time to research the fish I want...
The fish will stay in the QT with a piece of PVC "cave" and a 30watt NO bulb for observation for Three to Four weeks...as long as everything is going well I'll just do a 5 gal WC with main tank water once or twice a week during this period.
If the fish comes in with problems or someone in main tank need "treatment" the QT becomes a HT...
If the problem is ICK I'll use basically the same setup and do 'hypo salinity/raised temp treatment...
If the problem is bacterial or fungus in nature and needs 'Meds/copper" to treat...(i've never needed copper since switching to "hypo') I'll pull the Emperor off and use the substrate from the baskets/fuge in a "corner type" bubble filter...because I don't want to contaminate the bio wheels/filter.
Doing the "meds" according to directions (or sometimes modified directions from others with "experience") generally antibiotics which play "havoc' on the biological filtration...(which is why you don't want to "treat" in the main tank)... I'll do daily 5 gal WC with 'aged/aerated new saltwater or main tank water if the tank is healthy.
When the patient is healthy I'll tear the tank down wash it out and be "ready' for the next adventure...the "contaminated substrat gets tossed.
Since starting this procedure (with several fish)I have not had a sick fish or any recurrent "ick" outbreaks in the main tank.
I once had a Wrasse that after QT was still too small to put in the main tank with the "big boys" so I added a sand floor and some LR to make him comfortable, turning the QT into a "Hotel tank" for three months while he put on some size.
HTH