One way window films are readily available at low cost from home centers and mail order. They are cheaper than aquarium background and I am surprised no aquascapers reported using it as a mirror background. It will capture light by reflection thereby increasing light intensity for plant growth.
If you tape it on the front and sides, it will provide a one way see through glass in, and a reflective mirror (for fish) to see out, but only during photo period when the light inside the tank is brighter than outside. Off photo period, the one way effect is nullified.
Even without the film, glass provides partial one way effect during photo period. The one way effect can affect some fish behavior, as I have observed behavior change during and off photo periods. I keep cichlid which naturally establish a pecking order. I have witnessed a few times that the pecking order is disrupted when the light is on and off, for instance, the top fish can become the under fish and vise versa with a flip of the light switch. Other times I observed fish engage in fight when the light is out, but restore peace in accordance with the established hierarchy when the light is on. No wonder sometime my fish emerged with torn fin from the dark despite behaving peacefully when the light is on.