You have to take all fish out of your display tank and QT them all. Then leave tank farrow.
Depending on what type of fish you have.
Cooper vs chloroquine phosphate would be my recommendation. For tangs you can use either. CP is less harsh on the fish. No safe for all wrasses and certain other fish
Good luck
I had a small purple tang in my 65 gallon. Perfectly healthy with zero sign of disease. After a couple of years the purple got to large so I decided to catch her and put into my eel tank. Anyways, after finally catching her and putting into larger tank, my purple tang was WHITE! Absolutely covered in ick. What did i do? Nothing! Tang never stopped eating and in a month it was perfect again. No other fish were infected or my eels. All tanks have ick, keep your fish happy and eating and they will naturally beat it or die. To pull all your fish out is crazy.
Now velvet is another story. That **** wiped my tank out 6 years ago.