You really don't have to buy another background - you can just paint the back of your tank!
And as Sweetdawn said - on the OUTSIDE, of course
If you'd prefer the ability to change colors, then you can get craft paper - wrapping paper - or paint some newsprint in the color you'd like it to be right now.
Laminate it once it's dry, and then use velcro tape to fasten it to the back of your tank.
If you really really really still want to buy something - then ***** and the like sell rolls of plain blue background by the foot.
Let us know what you decide to do! :joy: