I really don't know what they may be coated with, but I'd speculate that they are safe, but may not last long under seawater before the thin metal layer corrodes and is no longer a mirror.
Yea looking forward to seeing what this looks like!
This actually makes me think that a black smoked mirror look would look good as a tank background. It could be attached from the outside. Anyway just thinking!
Thanks guys. Its going to be a bb sumpless 25 gallon with a lid. Im going the kiss principle. Will be the second tank in the house. Now i can use the entire stand area for all my supplements and testers. Rather than have them in the storage room. It will have a skimmer, and an ats.