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You can put a background on it! The main focus should be on the live rock or reef you have. Try to pick a background that won't take your attention. You can also paint the back the tank a solid colour!
IMO a plain black one is the best b/c it's attractive/makes colors pop a lot. You can cover the tank w/ newspaper and spray paint the back. It's really easy and doesn't take much effort. Krylon (from Walmart) works really nicely. I wouldn't suggest having a background other than a plain color. For example having a background of a reef..well why not have your own reef!
I agree with obscurereef. it wont stress them out. The main function of a background (IMHO) is to hide the tubes, plumbing, cords and other items that take away from the visual aspects of the tank.