Clownfish Ich?

[Ruth]

You will need to treat both fish if it is in fact ich that your fish has. If you can try and post a picture of the sick fish to see if someone can help you identify that it is ich.
If you read the link that Marie gave you it will give you the information on how to treat your fish but you have to keep in mind that you will have to leave your main tank fishless for a couple of months to allow the parasite to completely die off.