If one of your fish has parasites, Ich or Brook, ALL have the parasite even if they do not have any symptoms. Ich has a three stage life cycle. On the fish, in egg form in the substrate of your tank and in the water as swimmers looking for a fish host. The egg form can persist in your tanks substrata for up to 8 weeks. Brook multiplies by cell division, the new cells become swimmers looking for a host and can persist in the water for up to 4 weeks. If its Ich, then copper is the chemical of choice. If its Brook then only Formalin 37% will cure it. Formalin will also cure Ich. Copper will NOT cure Brook. You need to do the following.
1. Remove your fish to a QT tank and start giving them Formalin baths. One bath every other day for 10 days. This will cure the fish but NOT the infected tank they came out of.
2. The tank the fish came out of must remain fallow, fishless, for 8 weeks. Do NOT use anything from this tank on your QT tank such as filters etc.
3. If you do not wait long enough for the swimmers do die off in the main thank before you put fish back in, they will be reinfected and you'll have to start all over again.
There is no magic bullet or short cure for these parasites. And, unless you are sure of what you have, use Formalin since it will cure BOTH Ich and Brook. If you got this from introducing wild clownfish the probability is its Brooklynella since it is estimated that 80% of wild stock are infected with this parasite. In the open ocean, its not a problem but in a closed system like an aquarium, its fatal.