You could paint the back of the aquarium...might help. But then it would reduce the light in the room and also not let you see through the tank to the window. Oil based paints that stick to (cleaned!) glass are used, if I remember correctly. A cheap disposable foam brush(s) works well on glass. Easier to paint it before you fill it, but....oh well. Will require at least 2 coats.
There are also things you can buy to put on the glass...scenes, etc. Or reflective films. Some of the scenes/paint may not be reflective to the frequencies of light that you want to filter to reduce algae growth. Might take research, but not worth it unless you're interested.