You’re going to get a lot of differing opinions on this subject. I am personally QTing everything “wet” that comes into my tanks - fish, corals, and inverts. I have three 10 gallon QT/observation tanks going, one for fish, one for corals, and one for motile inverts.
All three tanks have an AquaClear 70 HOB with precycled media.
For fish, I’m following the R2R recommended medicated protocol
here. I placed this tank more than 10 feet away from any other tanks as some parasites can potentially be transferred from tank to tank by aerosol droplets of tank water in the air.
Corals get dipped and placed into their own observation tank for 76 days. The dipping helps with coral pests, and the 76 day wait helps to starve off any fish parasites the corals may be carrying. This is a bare tank with a frag rack and I’m lighting it with the same Kessil light I have on my display. I also have an ATO on this tank.
All my snails and hermits and what not went into a third tank for the 76 day hold. This tank I did add dry sand and a couple pieces of dry rock, and a ball of chaeto with ‘pods.
Some will say you don’t have to hold all these animals for the full 76 days, but I want to be a cautious as possible. Most would not want to put this much time and effort into this methodology, but I feel like I’m doing the right thing and I’m absolutely loving the process.
Good luck with whatever you choose to do!