I like the painted black back idea.Some of the photo backrounds are nice if you have a tank with a lot of plants/rocks/wood and match the photo to whats in the tank
You should go with the black background. Black color background really makes your fishes look more vibrant. And creats a nice look to it. I use black for my tanks and love the way it looks!
the tank I got is actually a 75 not a 55. My parents got it for me for xmas and they told me 55, but when I checked the box it said 75 I knew it seemed alittle big for a 55 lol
have a mirror cut for the back, it will make your 55 look bigger, and you can see what the fish are up to better, intercept thier escape plans and such! i did this years ago with a tank and really liked it, just have been leaning more to the diy backgrounds the last few years
if you'd like to prepare a real cool background, then do this: Get paper/cardboard the size of the back of your tank. Spray construction foam (polyurethane) on it, let cure. This way, you create a 3D background. Paint with oil-based paint and let the colors flow into each other. Use brown and green, but also bright yellow (sparingly). It'll look awesome. I do that for all of my tanks. It looks real natural.