How long has your aquarium been set up? The reason I'm asking is that you seem to have a measurable quantity of nitrites, which is not good, they should be at zero. Nitrites are quite toxic to fish. If the tank is brand new and hasn't been cycled properly then you will experience a large spike in ammonia and nitrites in the near future, which is no place for any fish to reside. When I set my tank up, it was 6 weeks before I introduced any livestock into it in order to get through the initial cycle. If you have cycled the tank prior to introducing the first fish, I'd definitely do a water change to work on getting nitrites down to zero.
Clownfish can choose rather odd things to use as their 'host', powerheads, overflows, and thermometers to name a few. As you add other fish that swim about and that aren't aggressive towards the clown, he may gain some confidence and venture out a bit more, or he may not. In the wild, clownfish pretty much live their entire life within a small box around their host, hence his obsession with one small area of space around the thermometer.