I think adding to mirror to a tank to "make it look bigger" is tacky because I have never seen a tank with a mirror on it and with the size tanks you have it would make no point trying to make it look bigger because they are already big and IMO it would insult the tank by having a mirror in it. I know if I got a big enough mirror to cover the back of my 55g it would just look ugly and I would suggest you to get a blue or black background, it will look alot better for the money you spent on the tanks.
