If it is ich, turn temp up a little and throw RID ICH in the tank. This is the safest treatment i have found for reef tanks. I got it from lfs. I paid under $7.00. I only used 2 treatments and everyone is healthy, fat, and happy. Turning the temp up, increases the ich cycle. I would treat the whole tank. The chances of the protozoans spreading in the tank are high. If 2 fish have, the parasite has had the chance to grow and spread. If you need help, feel free to give me a shout. Copper is also good if you do not have a reef tank. Treatment should be started immediatly. It is also good to know that ALL FISH ARE EXPOSED TO LOW LEVELS of the protzoan.