I went to Jo-Ann Fabrics and bought a shiny black material that I put sticky-back velcro on. Stuck the other part of the velcro onto the back of my tank near the hood (can't see it), and stuck the material to the tank that way. It worked great - and it's been that way for about a year and a half now. I like the way the shimmery black looks behind my plants, etc.
And it's easy to clean it. Un-velcro it and toss it in the laundry, then stick it back on after it's clean.
A lot of people get creative when it comes to backgrounds on their fish tanks. I'm sure a garbage bag could work too. Just depends on your taste.
Why don't you use a black cardboard poster that you can buy for about 50 cents at school supply stores? It would look a lot better than a trash bag I think
you can have whatever background you want... you could even experiment and make some of your own and see what they look like... personally my favoritess are plain metallic.