how to make a back ground

There are a lot of ways to add external backgrounds that don't cost much...I have painted the backs of tanks directly (before setting them up..hehe) and have painted plywood the same size as the back of the tank and the hob filters hold it in position. My wife's uncle is an artist...he painted several backgrounds on heavy duty poster board for his beautiful planted tank...and changes them to suit his mood! Years ago, you use to be able to buy this special paint that came in variuos colors - you would paint it on the back of the tank and it would dry in crystal-like patterns. If you want to get really hinky, you can build a sort of "shadow-box" the same dimensions as the back of the tank, about 4-5 inches deep, and add plastic plants, driftwood, rock, etc. to give the tank a sense of greater depth - good for cichlids!

if you wanna use styrafoam you just mold it and then cover it with qucikwall made by quikcrete, you use concrete dye to color it let it dry and cure before adding fish. check the ph first because the concrete will make the ph high. do a search theres a good writeup one of the cichlid boards; i just forgot where it was. if you want simple and want it to look good. i just spraypaint the back of the glass tank.

background

I have used dyed foam as a background for a show tank I did at a childrens hospital. Turned out fine for the setup of random colors.
I personally use wood or wrapping paper. Wrapping paper has been good to me for a long time, if you set it right. I dont suggest the shiny mirror kind if you are on a larger tank with curious or personable fish. I had an oscar that liked to look at himself in the shiny silver background, then ram his own image in the glass.
As long as its not toxic, if you imagine it you can probably pull it off.

www.thenaturefactory.com They have excellent backgrounds!

I went real cheep on a back ground... My wife had some Dark Turquoise fabric left over from a project she finished.

This is pretty cool also:

cork bark background

And there's always the famous

Moss Wall