Is this a reef or fish only system?
First step is test up your water parameters and try figure out what caused the break down.
Watch your temp and specific gravity as well as the usual water chemistry parameters.
Then probably a water change to clean things up.
Start feeding high quality frozen food soaked in Garlic Guard if they are still eating.
If there are corals and inverts, medication options are limited/nonexistent.
Isolation in a QT system with hyposalinity, copper, or other meds is usually best.
If it's fish only, remove carbon, Polyfilters, turn off Protien skimmers, and medicate in tank or hyposalinity.
Before treating, a pic of the fish and parasites would be helpful for selecting the right meds.