Normal behavior for Pangasius. I had mine do this in my 300g at 4" size. Stayed in the corner always. I've seen them swim in circles with super fast speed for hours sometimes in figure 8 on one end of he tank. They never just swam around like the other fish when they were small. Not until they reached around 10"+ size did they begin to use the whole tank. But still now at over 20" they like to get vertical against the glass and just hover there.
