I thought I have heard of people putting oval shaped mirrors on the bottom of their aquarium to simulate the appearance of a lake...does this work? and if so, what kind of materials would be non-toxic to the fish?
If the bottom of your aquarium is glass, just attach the mirror to the underside of the aquarium, then you don't have to worry about mirror compatibility at all because it would be outside of the aquarium. Mirrors are just glass with a reflective backing applied to them, so what's an extra bit of glass?