I have the back of my 150 mirrored. The fish do not pay it any attention at all. This tank started as a saltwater tank and now is fresh. When it was salt, the rocks were about 6 inches from the back and we were actually able to observe the fish that typically "hid". It also gave the tank an appearance of more depth. (obviously)
Now that the tank is fresh, I like it even more, the plants reflections make it look very heavily planted when in fact, it isnt. As I already mentioned, the fish could care less. The tank has huge angels, a blackghost and a very large silver dollar, along with some smaller incidental fish.