Whats on your back?

Plain black in all of my tanks. I find it draws your attention to what's in the tank (fish, plants, etc) instead of the background.

I like your idea of a mirrored background, sounds cool.
 
I really didn't like the backgrounds offered at the fish store, so I scanned in a grayscale image of large Jellyfish, colored it with a pleasing blue tint and taped it to the back of my tank. I am planning on changing it quite frequently, maybe give the fish some art history education. Some Monet's, some Picassos... Photoshop is so fun.
 
The tank at work has a sheet of white paper--legal size, to be exact--to block the light from a nearby window. All my other tanks do not have any backing on them. Most are against a white wall, and for the 2 newest setups, they are set so I can see them from two sides.
 
I just recently got a background poster made. It shows the head of a white shark with a gaping mouth and big white teeth coming up from the depths. Looks like it's about to crash through the back aquarium glass. No reactions from the fish so far but I love it.
 
Mines just blue. It was black but my fish was getting mean to the otehr fish so i changed it. Then they were both scared to come out of there caves so it worked pretty well.
 
Solid black fish store bought background. I used 'cooking spray' (Pam) to make it stick to the back of the tank.
 
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