In our tanks we decided after some consideration for a blue background. Blue has the positive attribute to give a much stronger effect of depth, especially compared with the use of reef ceramics. In our tanks, which are just 60cm deep, the optical depth of course plays a important role for the subjective look of the scape. Some aquarists also use reef ceramics as decorating for the back side. This has advantages (eg more space to settle corals) but the tank looks finally much less deep and doesn’t seem to be endless blue, as we know it from the coral reef (pictures).
As you can see on my tank pictures, subjectively there is a marked difference between the right side (black glass) and the background of the remaining pool. Although the black glass side reaches only 9cm into the tank, the effect on the subjective look is much stronger. In addition, the coral and fish colors seem to be much brighter and more intensively. The blue background creates a sensational contrast, especially with the color red, orange and yellow.
Now there are some hobbyists who paint the back wall just blue. However, we have decided on a self-adhesive film (color: S5299b sky blue, available here).
The application is relatively simple: The adhesive surface is first rubbed with a little bit of soapy water and then applied to the glass surface. The water has the effect that the film will not stick right away but can be moved on the right place. With a microfiber cloth or possibly a plastic spatula, the water then rinse out and the film is fixed on its position. Air bubbles can be pierced with a tiny needle and then pushed away. I have sealed the edges with silicone, so that no water can run behind.
Particular care should be taken that no dust, hair, etc. get onto the adhesive surface. At one tank, we’d accidentally “incorporated” a black hair that now look somewhat disturbing.
To obtain the optical depth, the back side should be always free from algae. I use a magnet cleaner blade to clean it, as well as the front glass, every 2 days. My tank stands in around 10cm distance from the wall, so I can get behind it with my arms while cleaning.