A Backdrop for Behind the Aquarium
Your backdrop can be anything from dark fabric to a white wall. You will be making some choices about whether your background will be in focus or not later, but that will be a primary concern about how photogenic the backdrop itself needs to be, and therefore how much thought you need to put in to it.
I am currently using a small bit of Duvetyne, which is used for theater sets. It cost me $12 USD for 55"x30" (140x76cm). It's fine, but I would recommend you blur the background a bit as it is not a particularly attractive fabric. Velvet would be better if I wanted the background to be in focus.