I always chuckle when I read people suggesting to line tank with black paper, as I use to have a black background for Blue...yet when it got darker outside, and inside depended more on lighting, Blue seemed to be able to see his reflection more from the Black "wall". He would "pace" back and forth along only that "wall", fairing & puffing up his fins. I removed the black, and he seemed to calm down.
I would say if he dose it the entire time, and you have no kind of back grounds for tank, either the watts are too high [causing reflection / sheen on window] or he's just not use to the light yet.
I bet its more with the light being bright, and he see's a small bit or reflection of himself in window. Try either getting lower watt light, or use a lower watt desk lamp next to tank, and see if any changes. Wouldn't hurt as they're use to low light anyway in nature.