Off the top of my head I don't know of any condition that exactly matches those symptoms, unless the appearance is a bit deceptive. Could it possibly be strings of excess slime coat that you're seeing? One possibility could be Brooklynella, which often causes sliming and is common in clowns. It usually causes a milky appearance to patches of the skin, so perhaps that's what you're seeing as faded color? Do a google image search for the disease and see if anything looks similar to your fish. If it is Brooklynella, then formalin is the most effective treatment and absolutely shouldn't be done in the tank.
Even if it's not Brooklynella, if you have any invertebrates in the tank or plan to have them at any point then don't add any medications to the main tank. Although there are some marine meds on the market that claim to be invert-safe, they are also generally ineffective. Medications that actually work against things like ich, brook, and other parasites are a risk for harming invertebrates. If you need to set up a temporary hospital container for the fish I doubt the condo association could fault you for a small one given that it's not intended as a permanent fixture. In a worst case, you could even use a couple gallon bucket for a treatment as long as you change the water daily and don't reuse the bucket for anything to do with the main tank later.