Below is a writeup I did for another post, it may be helpful to your situation. The first thing you need to do is determine what your fish have, one of the various forms of Ich or Brook (Brooklynella).
Read up on Brook and Ich. Brooklynella is a parasite that has very similar symptoms to Ich which includes Marine Velvet are also parasites. Brook is treated with Formalin and Ich is treated with copper. Some of the symptoms are rapid swimming, swimming at the top of the tank and gasping for air, fading color especially in the head area, sometimes spots on the body but not always, sometimes lesions on the body but not always. Slim on the body and head. Brook can take your fish especially if its stressed out in a matter of days, Ich takes a little longer but it too will kill. If one fish in your tank has either of these parasites then ALL of you fish have the parasite even if they exhibit no symptoms and have to be treated or eventually they will die. That means setting up a treatment tank with copper in it and giving them Formalin dips. The tank the fish came out of also has the parasite in it and all of the fish have to be removed from the tank for a six to eight week period to stop the life cycle of the Ich parasite or the "swimmers" of the Brook parasite. Ich eggs are in the bottom of your tank and can lay there dormant for up to six weeks. Brook "swimmers" are in your water and live for up to four weeks without a host. When Ich eggs hatch they have to find a fish host or they will die. After six to eight weeks the tank is safe for new occupants. Get on Google and read and read and read some more about these parasites and how to treat them and do it now before you lose all of your clowns and your other fish. Remember there is no one day cure, no one chemical cure; these parasites are difficult to get rid of so from now one, QT any new arrivals for at least 3 weeks or you may get stung again.
