There is a very good chance that all of them are sick.
When I wrote gravel, I was really meaning any substrate, whether it is sand, cc
, aragonite. Beneficial bacteria live in the substrates and treating your main tank with copper will kill your beneficial bacteria and any invertabrets in it. However if you plan on having a fish only tank you can treat the main tank with copper, just remember anything in the tank at that time can NEVER be used in a reef tank.
If you treat your fish in a quarantine tank for a 2 - 3 weeks, in that time the parasite will not beable to find a host and eventually die.
P.S. while quarantining your fish make sure to do a 20-30% water change in your main tank.
Hope this helps and feel free to ask any other questions.