What to put on the back of tank to make it black?

Spray paint works. They also have plastic backing that are made for fish tanks that you can pick up at LFS. 

I like to spray paint the back. You can do window tint, but it doesnt always come out perfect. My 90 was tinted on the sides and back with static cling window tint. I have found the kind from sears has the best color, the walmart crap is too purple.

if you want it to be easy use spray paint... If you want it to look really cool, use tint. The other nice thing about tint is you can see in really well, but the water makes it nearly impossible to see out, so if you have a fish that likes to hang out in the corner you can watch him without him knowing you are there. It also seems to hold just a bit more light in the tank with the sides tinted.

I use shelf liner as well.

Thanks for all of the feed back I will look into the tinting and also the drawer liner sounds like a good start I wanted to use something that you could take off if needed thanks

The best stuff I use is from a sign shop, vinyl. Its wasy to stick on and they usually have scraps laying around.

All other things being equal, try to stay away form painting the tank. James Fatheree (who spoke at a recent WMAS meeting) wrote an article in May 2009's Tropical Fish Hobbyist. The gist of it was that painting the outside of a tank will reduce the amount of light reflected to the lower part of the tank. What happens is that light reflects off both the inside surface of the glass and also againts the inner side of the outer edge of the glass. Painting the glass will keep light from reflecting off that second surface.

I hope that made sense. If you have access to TFH past issues, it's a good read.

In doing research for the article, they found that anything sealed to the outside of the tank, he tried different colors of paint and a wet piece of plastic with the bubbles pushed out to seal them together, could reduce the light in the back part of the tank by as much as 30%. I'd imagine that something like window tint would do the same thing, but I'm not sure.

One more thing to consider, if large areas of the back glass are covered by rocks, they won't be reflecting at all anyway, so it might not matter what you do. But most of the reflection takes place at the higher portions of the glass, and those are usually covered with less rock than the bottom portion.

Hey thanks guys my final decision was to go to petco and get some black tank backing and I double sided taped it to the upper and lower rim of the tank so that you cant see the tape from the front of the tank It looks good I Think I will post a pick as soon as the sand in the tank settles lol