I had mirror on the back of my 150. What we did was take the "door mirrors" that you can get at walmart for 5.99 each, took two of them and got some velcro. Put the velcro on each corner, cut small enough so that it stayed where the tank seems were, you never saw it and the meet was perfect between the two mirrors.
They were layed left to right and then the next one just above it. This way, we could take them off to periodically clean the glass...no matter how hard you try, you WILL get water on the back glass.
No fish that Ihad ever gave it a second look.