To answer your original question:
Try painting the background or adding some type of background. I say black would look good.
Maybe a nice piece of driftwood and some river rocks on the ground to make it look more natural.
You've been advised on your stock and what they require so I won't go into that. I'll just address your original questions
Also, you can but construction paper and put it behind the tank to see what colors you like but it will his the wires and wall and help make your tank and fish more of a focal point. When looking at your tank now I see a filter and wires. It kinda distracts you from the tank.
Side note. Try rotating your pictures, there sideways
Also try using all the same type of rock instead of all different kinds, you might like that look.