Since the cleaning solution will be disposed of, you can use regular tap water. Most people use white, distilled vinegar for mild cleaning but for the tough stuff, a dillutided mixture of water and Muriatic acid can be used with caution. As far as the back of the tank goes, some people like to color it with some type of colored plastic paper, black is a popular choice, blue is another, others just leave it alone to age and allow it to be populated by coraline algae and other marine organizms which is what I prefer.