Yup, you're over populated way much!! This will help a sick fish get more sick. My advice is treat the clown in a separate tank, and when it's better, make sure it goes in a less crowded 50g (give away some fish, dude!!)
A generic anti-fungal med will probably help, although it could be fluffy bacteria. Check the packaging for symptoms and treat accordingly. Did I mention you should give away some fish?
