I had the same concerns when I added my 2 perculas. The first day or two, they would only swim in one small area of the tank, probably only a foot cubed.
Now, they've got a new wierd swimming style. At night, each of them goes to an opposite corner of the tank. One swims almost vertically. I was really worried he was stressed, but there are no other fish in the tank (besides the other perc), my water tests came out 0/0/10, and he swims perfectly normal throughout the tank with the lights on.
With my limited experience, I'd say you've just got a loony clown like mine.