My only take on this is that the sun is in their eyes at this time of morning as the window to the right of them is fully open to light and the sun is passing in front of the window at exactly that time in the morning and streams a lot of light across the front of the tank. When the sun gets in at a certain angle they angle their bodies by 10 degrees to avoid the direct sunlight streaming into their right eye.
Does this seem like a rational explanation? Anyone else think of a better explanation?
Has this been observed by others?
I will try to take a picture of this tomorrow morning.
Usil
If I am reading this correctly, they are leaning toward the light? If so, I have observed a similar behavior of fish orienting themselves to light on night dives. I was introduced to what my dive instructor referred to as "fish tipping" when I did my first night dive. We would find a sleeping fish, shine a flashlight directly down on it then very slowly tilt the light to one side. Many times the fish will keep its back to the light and we could get them to tip over onto their side. Sunfish seemed particularly prone to this behavior, the majority of bluegills and rockbass we tried it on would tip.
They may be orienting themselves to the light, in a natural setting the light is always going to come from the surface, and while it may not come straight down, it won't come as much from the side as it does through a window and the side of the aquarium.
On the other hand, I also had a spotfin shiner that lost an eye. When I would come home from work late at night and turn on the light, it would swim fully on its side next to the bottom with the eye facing down, presumably to block the bright light. After several minutes as it became accustomed to the light it would swim in a normal position. Your fish, as you suggested, might just be trying to keep the bright light out of their eyes as well.