Here is how I quarantine when I get livestock that seems healthy (like my clowns) this is NOT FOR SCALELESS FISH LIKE MANDARINS OR SCOOTER BLENNYS.
1st I treat for small issues like gill flukes and worms using seachem para guard. I do two weeks treating up to 3x a week (if they have flukes, with the initial dose they may freak out a little when flukes start popping off them. I know it’s overkill but it’s a way I can ensure to get ALL of a flukes cycle.
2nd. I treat for a week with metroplex in their food to avoid any secondary infections/ finish off the worms and issues like Uronema
3rd. Now that they are fluke free and dewormed, I run copper. You CANNOT start at a full dose. You will kill your fish. You have to work up to the recommended dose. USE A GOOD COPPER TEST KIT. ITS BETTER TO TEST MORE THAN LESS. You have to keep the copper at certain measurements for a set time and do water changes in order to slowly wean them down.
Like I said, this is the way i do it for a seemingly healthy fish (others may feel its overkill but I even quarantine my crabs). If you are concerned yours has velvet you need to act quickly and start backwards from 3 to 1.