If by "treat for everything" you're planning on using medications like copper specifically, the problem is that the copper interacts with the rock and sand, and is very difficult (though reportedly not impossible) to remove from the rocks after treatment. Typically, medicated QT tanks are bare, with no rock or sand, and only PVC fittings for hiding spots for the fish.
Although some have said that they have been able to pull copper out of rocks and sand by running
Cuprisorb to me it seems like you'd spend a lot of money and time and effort doing it that way. It may in fact be cheaper and easier to setup a QT tank.
QT tanks can be very simple, Aqueon tanks are 50% off right now at PetCo, and people have even used plastic storage totes and buckets in a pinch. Some PVC hides, bubbling sponge type filter or a HOB, and a heater, and not much else. No light needed.
There's more than one way to QT fish by using medications, or there's something called the Tank Transfer Method (TTM) that doesn't require medications, but does work to reduce fish parasite. Some reefers do nothing but observe the fish to see if disease symptoms become noticeable. The problem with that is that disease may be latent or suppressed, but can come back with a vengeance when given the right opportunity, like if your fish get stressed for any reason.
I personally follow this guide. The person that wrote and maintains this thread is a professional in Public Aquaria, and is available right here on R2R to ask questions:
https://www.reef2reef.com/threads/current-quarantine-protocol.825055/
You can setup and then breakdown a fish QT tank as needed - no need to have a whole tank running all the time for fish.
Tip: keep some extra bio media in your display tank's sump or filter section that you can pull out and use in a QT if/when you ever need it. This way you have basically an instantly cycled QT tank whenever you need one.
I personally do my best to QT anything "wet" including fish, corals and motile (moving) inverts such as crabs, snails, shrimp, etc. I have a separate tank for the corals and inverts, I hold all new arrivals for 45-60 days before introducing them to my main tanks. This is non-medicated, and over 10 feet away from my fish QT tank because - believe it or not - fish some parasites are so small they can become aerosolized and travel via a droplet of water through the air to neighboring tanks. Yikes!
Good luck with whatever you decide!
