Just a couple of weeks ago, my brother decided observation being he got the fish from a LFS that keeps its fish in 2.40-2.5 copper. Ended up getting another fish from another shop a day later. I told him to dose copper and get the QT going ich ended up on the fish, and lost time, about a week.
Here are the facts. Unless you get the fish from a QT vendor, do not wait! Why wait? Do you know if the fish has ich, velvet, brook, flukes 100% or not? And if they do but takes a week to show you just lost time.
In a time frame where we see diseases more prevalent than ever why lose time? And risk the fish to get more of an infection? Stress is also a factor and many LFS’s now under run some kind of suppressant meds, or low salinity to suppress the diseases. Thus most fish can become carriers.
Just my two cents. Get the fish in copper as soon as you can.