you can buy black backgrounds from the store or try something like cloth or poster board. i just ended up draining my tank and dragged it out a bit to spray paint it since i figured it would look much nicer. moving the tank was actually pretty easy, sniffing in the paint while indoors wasn't fun though
Check a craft store or wally world for some foil/mylar wrapping paper.
I would stay away from paper or felt that could hold water and get moldy or smelly.
The weed cloth people use in gardens works.Might have to double it up depending on how dense t is.
You can get limo tint cheap on ebay.Not easy to get on smooth though.