I really like the look of the 3D fake rock foam backgrounds. The downside is that they're pricey and they cover the back so you can't suction cup anything to the back panel. My second choice is a black background since it has a very clean and non-intrusive look, as well as the benefit of helping hide black outflow pipes, spraybars, wave makers, and airline tubing. You want the focus on the fish, not the equipment. Some people paint the back of their tanks, some use laminated backgrounds, some don't use anything at all. I had a bit of a challenge with my 120 gallon because I didn't want to spend a lot on the background, yet the 3D foam rock background was crazy expensive, the laminated sheets in that size were still pricey (and usually don't look that great, Imo), and I didn't want to mess up potential resale value by painting it. So, I came up with a bit of an unusual solution. I covered the back with Plasti Dip, a rubber that can be bought in an aerosol spray can or dipping container. It took a bit of masking and three spray cans, but for $18 I got a solid background that is waterproof and can be easily peeled right back off whenever I want. The best part is that it comes in a variety of colors and can be bought at Lowes for $6 a can. (I've found it also works well for electrical insulating, tool handle grips, and pretty much anything that needs to be waterproofed or protection coated.)
