Hi,
It's been my experience that clownfish will get very lonely and lethargic if left alone in a tank. I kept a false perc alone in a 20 gallon for a few months and he didn't move much. He would just stay by his house, occasionally swimming to the top. He got more and more lethargic as time went on. We finally decided to add a friend for him and got a very small flase perc. He (now a she) really perked up and now they swim all over the tank. She also about doubled in size within months after adding the young clown. I think clowns are social fish and need company or they get bored, so I would add another one with it from the start if possible.
-Steve