any time a clown is in the sand is usually a bad sign. Watch for labored breathing. If its breathing hard, you might want to take it out to QT in a separate tank. Also be on the look out for weird skin abnormalities. Slime coat coming off, red patches, white spots on fish, fish swimming into current.
Methylene blue is usually used to help with ammonia burn and considered a first step in most initial fish treatments. Its nice on keep on hand in case of fish issues. It stains everything so keep that in mind.
This thread contains links to commonly encountered saltwater fish diseases. Click on the blue links below for more detailed info. :) Bacterial Infections Bacterial Tufts & Viral Nodules Black Ich (turbellarians) Brooklynella hostilis Clownfish Hyper-Melanization Eye Problems Flukes...
humble.fish
Humble fish has a pretty nice website on fish diseases.