This is one of the reasons I always recommend hyposalinity in cases of confirmed
ich. It does not affect the biofilter in the least and makes the ease of the process quite painless. If I remember correctly, you (us?) were not able to tell if this was ich or velvet. Unfortunately in that case, copper must be used as hypo has a very limited affect on velvet.
Placing the fish back in the main to fend for themselves or leaving them in the QT
would not have the same result. If placed back in the main there is a very good chance they will succumb to the parasite and die. If left in the QT
and treatment is continued coupled with the needed (twice daily in some cases) water changes, they have a much higher chance of survival. It has not yet been a week and the needed treatment time is 21 days using copper.