Hello and thank you for your inquiry.
Years ago (maybe decades???) tank manufacturers actually made tanks with mirror backgrounds. They did note that these aquariums were not safe for saltwater use due to the corrosive nature of saltwater.
As long as what you use for the mirror finish is "safe" and will not leach anything in to the tank it should be ok for a freshwater tank, but I would not recommend it for a saltwater tank.
Most people have found using a black or blue background is a better choice for showing off the inhabitants of the tank.
If there is anything else we can do for you please let us know.
Marine Depot Customer Service