Surely already mentioned, but use a QT. Understand what it is for.
And understand that a QT is not a tank you can have fish in for 3 weeks and be ready to move them into the DT only to have a family member add a bag of new fish and think I could still move the 1st ones because they were in there for 3 weeks (Forgot that the new fish could have brought sickness with them).
Unbelievable but true story.
Agree with fish sitter problems. Let the fish be hungry for a week!