first i have a question, are you planning on putting the background in the tank? or on the outside of the glass on the back of the tank? if you are putting it on the inside use something from an aquatics store. if you are planning on doing something on the outside there are many things you can use, diff types of woods, backgrounds you can buy at the local pet store, using diff fabrics, or even painting the outside of the glass, the last one looks GREAT, depending on the size of the tank you are painting, you can use anything from glossy latex, to acrylics, I've painted the outside of several tanks, one thing you must remember is that when painting the back of a fish tank, do your foreground first then work towards your background, I've done several with marbling effects, do your lighter colors first, then work in the darker colors, if you want a natural looking background remember to paint in your leafy tendrils first, using a wider brush like a half inch, start flat at the bottom then as you work up to the top you twist the brush slightly to one side then slightly to the other this will give the leafy tendrils a completely natural look, if you want really natural looking tendrils you can dip your brush in the darker green and then dip one corner of your brush into a lighter green or greenish yellow to get a natural lighting effect with the paint, for the rocks and such that you may want to have in the back ground you can use sponges, dip the natural sponge lightly in a brownish gray color or whatever color you choose and then push it lightly to the glass, keep pushing until you get the effect that you want. and remember when you paint on glass most paints can be easily removed with a bit of watered down paint thinner or mineral spirits... i use acrylics as well as semi and high gloss room paints and none of these seem to effect the fish in anyway at all. if you have a local art studio you may be able to commission one of the artist to paint the back of the tank exactly how you would like... it may be expensive but well worth the one of a kind background that no one else has... good luck i hope this helps.