When I got my used 55, it came with a mirror background painted on so I left it.
My fish do fight themselves in the mirror a lot, but on the flip side, isn't that better than them nipping each other? I can double the amount of fish that are in my tank to spread out aggression, without adding any fish lol.
There was a study done on this, I can't find it right now tho. the study tried to determine the affect of a mirror background, and how the fish interperted their reflection. Try to google it.
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