My thoughts and what has worked for me.
1st off there is quarantine and treatment.
I choose to treat my new fish regardless. I don't believe my observation alone will detect possible infections with the fish all the time. Ich for example could be in the gills and you would not see it on the fish.
I do my best to have a sponge filter going to help keep ammonia in check but I have gone without one before with lots of water changes and checks for ammonia.
I treat my fish for 5-6 weeks before they go into the display. The waiting is the hardest part.
I tried Hyposalinity at the beginning but found keeping my new prepared salt water at the low level of 1.009 for water changes in the QT tank left me with no water to change out for the Display tank. When I was just starting out I had the QT tank running at least 4 mths in a row. The option would be to have prepared salt water in two batches but that was not going to work for me with the amount of water changes and storage of fresh salt water.
I changed water min every 3 days while running Hypo method.
Now I start with Prazipro for 1 week using water from my display tank as I trust its good, and the fish will end up there anyway.
Next I dose Cupramine for 4 weeks. when I do a water change I add enough Cupramine back in to keep the level in check to beat Ich. I use both water from the display tank and new fresh salt water. I change 25% of the water at least twice a week. If I am changing water from the display I use that to save water overall. 1 more week observation with a few fresh water changes from the display to reduce the Cupramine and done! Happy Fish...Happy Life
This process is working well for me. I tried QT Hypo at first and lost some fish so thought I would bypass this QT stuff and put the fish right in the display.
Well that only worked for a while and the display got infected with Ich. (A Tang of course appeared to bring it with him) I had to pull all the fish out and QT leaving the display Fallow for 10 weeks. Now that was pain full and I lost some of the fish in QT. Never again will I not QT and treat the fish , as corals grow its a real PITB to try and get fish out of the display.If they are sick it can ruin what you worked so hard on, plus the risk of losing your favorite fish in the display.
This is not perfect I'm sure but its working for me.
If a 20gal or smaller tank maybe QT is a bigger deal than just putting the fish in the tank and I might myself even risk it in a small display but not in larger tanks to big a risk and cost.