Great question. I'm not a fan of current copper-based QT and prefer TTM.
With @Jay Hemdal recent updated QT protocol and @Humblefish Hybrid TTM protocol, we have two tools to increase our success for keeping healthy fish and disease out of our display tanks. As a Reefer, I'm always looking to use the right tool for job and preferably the easiest that ensures...
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For me personally, at this point I only buy QT'd fish or fish from a local reefer who QT's his fish and has a clean tank.
QT'd fish usually go into observation tank (OT) for a week or two. If I find issues I come up with a treatment plan. If it's an expert-level fish with special needs, it probably goes directly into DT.
If I can't get a QT'd fish, I use a slightly modified version of the TTM.
Transfer 1 - get eating and observe; if I see issues then come up with treatment plan.
Transfer 2 - prazipro last 24hrs
Transfer 3 - observe
Transfer 4 - prazipro last 24hrs
I'll also dip and feed medicated foods depending on the fish's observed issues and eating habits.
For coral, I'm less concerned. Again, I only buy from reputable sources. However, each coral is inspected, cleaned, re-mounted, and dipped in Beyer Complete before going into DT. Anything suspect goes into OT.
That's pretty much it. So 12 day TTM or OT for fish. Clean and dip for coral.