Well so far several members have seen my new Qt system. I want to share some of my experience thus far. Its a 200 gallons of dedicated SW filtration and tanks areas for Qt fish for my other tanks. A local friends who has a license has been trading me credit for helping him build some fish related systems stuff and i have been ordering bread n butter reef fish for my systems in exchange.
Immediately I will admit i think the best way to get a fish is honestly to go to a local store pay for the fish and have them hold the fish for 2 weeks for you. That honestly is going to be the most cost effective way to get a new fish and basically leaves you with bringing him home and holding him for a few more weeks to a month in a small 20 gallons ish stable dedicated tank to finish qt. In 2 weeks he will be mostly treated eating or dead from most of the usual issues. Leaving you with things none could really do anything about to take as risk. That or pay top dollar for fish from existing systems that have been in captivity for months or years.

My personal results after five years of buying fish from local stores was leaving me with as much as 50% losses due to all the things that happen in this hobby. My issues included

Death from suspected sickness
Death from known sickness
Death from non transition to new diet "starvation"
Death from predation "eaten"
Death from user error which includes jumping out, wave makers and other forms of suicide.
The infamous mystery losses with no explanation, just gone.

I had enough spending money on fish. I honestly need a fair selection of wrasses tangs and bennies n so forth just to keep my corals alive and healthy long term so i started a QT system plan.

The system is a 200 gallon total volume 6 tank "some subdivided into smaller areas", MARS system I got used and refitted with new fittings as needed.
Its basically what you see at pet co or walmart. It has dedicated heat, UV lights, and a common sump, with a large bio wheel and prefilter. I monitor PH and salinity and temp. lights run 10 hours a day I feed up to 2x per day. the idea is minimal extra cost day to day and minimal things to fail but still provide very stable conditions for QT of groups of 20 or so fish at a time. This covers my needs for the different tank and room to collect some new species to try as well and have a small reserve of maintenance fish.

There are alot of things to consider when trying to medicate and QT new fish that all add up to losses and instability compared to a stable existing reef system. But we all should know you can't afford to drop in a sick fish in our established tank. In the future id like to establish a full Coral QT as well.

When you medicate fish "with anything I think is honestly effective" you create several problems.
1=you kill the biological system that keep the tank stable
2=you stress the fish
3=you don't effectively treat the fish in time to save them with out stressing them to death from all the other factors.
4= other issues that add up here n there as with all other reef tanks.

There is known complication between medications and the time they become effective Vs how long the fish will survive from the illness or the medication.
Some of the things i think i have figured out is hypo salinity "which is like almost 50% salt water/ fresh" has to be way way down there to really offer any good against bad stuff 28 micro sirens VS 53 in normal SW for 30 days. Which does not favor stable biological filtration. Fish do seem to handle it ok but as with everything going down slowly is important.

Copper treatment is effective but same as above has to be done slowly and killed biological system and inverts, and has to then be undone afterwards. it has a limited life span in the water column some say 30 days ish i think its a bit longer but not sure yet.

Very sick fish need immediate identification and treatment which so far is a fresh water dip in ph adjusted highly turbulent aerated treatment for several minutes to kill many of the issues that may kill the fish before the other methods can be effective.

all this stuff still leads to losses. the learning curve is sharp.
to be honest your dealing with fish that just came from the ocean and already have alot things working adjacent them.

Keys things to consider are

Minimize stress from the moment you get ahold of the new comers till there at an effective level of treatment. Lots of little way to do this and the add up from how there captures at the store and the bag there transported in. how they rid in the car. how there introduced into the new water.
Identify sickness n stress rapidly and treat correctly.
Get the fish eating and behaving normally asap. It take a few days for them to get used to people around the tank food coming from a hand and confinement. hiding places calm tank mates and stability help this alot.

Many of these steps are conflicting which means you need to find a balance between stressing the fish and minimizing loss.

There is a ton of miss information on the internet. Some fish are just going to die. There too far gone or something is out of your control.
There will be mistakes on your part that may be fatal. The information you have helps you make better decisions on how to move forward.

If anyone has any feed back please post it. I think were going to try n do a talk on this when we have a future meeting at my place and go over anything additional i have learned from my experience.