Not sure if painting the sides would help, but it couldnt hurt. I would give the fish a week more or so to acclimate into the tank. Fish can be skittish when put in new environments.
dreaded flashbacks of college physics class lolThe fish should not be able to see himself in the sides of the tank unless you put a mirror against the tank wall.
When a fish looks at the glass head-on it is clear and they see out into the surrounding room.
The only time you can see a reflection in glass-water boundaries is when you are viewing them at a very steep angle. Such as when you look through the front of the tank at the sides, they look like a mirror. Looking through the sides, the front and back do the same. The far side is crystal clear.
If you're looking at a steep enough angle to see a reflection, you can't see your own reflection. The view is of things past you, not back in your direction. This is why you see the back wall of your tank reflected in the sides, not your own face.
Painting the sides, or applying a plastic film won't change the reflections, because they are occurring on the inside of the tank. It is the difference in refractive index between glass and water causing the reflection. A paint or film will prevent the fish from seeing outside the tank, but won't stop the high-angle mirror effect.