Yeah, the way mirrors work, the light would have to pass clean through the pane of glass and then reflect back through the pane of glass again, this will hinder the reflectivity a bit and make it not worth as a reflector, it's all going to be about the more aggressive fish, like a maroon clown spotting another mean aggressive maroon clown reflection and going nuts on it. And tons of the "reef safe" fish we purchase in this hobby are fine in a tank and with tankmates, but with same or even similar species, will fight over territory, even if just a reflection, If your fish don't mind the reflections, it would definatly look cool and I wouldn't worry about reflecting too much light in the tank at all.