I use backing paper to great effect, Painting is to permanent for me. The trick to using backing paper is to keep it off the glass, have a space of about half an inch between the glass and the paper. I use a backing paper that goes from a deep dark navy blue at the bottom to a more brighter ocean blue at the top. Blue brings out more colour pigments of whats inside the tank, (hence the reason for blue screens in photography and film work.) Give this a go as a temporary measure to get a feel for what you like, rather than painting your tank and not liking the outcome.