I don't see why you couldn't unless its a DYI then it would have to leach some. Im also pretty sure you may have to pull everything out of the tank in order to place it into the tank. *** never done it Im just putting in my 2 cents.
I am about to do it myself. That is when I finish my background. My plan is to move the rockwork to the front of the tank to clear the back. Then move the sand away from the back as much as possible and place the backgroud on the bottom. The top will rest against a glass strip in the tank so it wont' float out of the tank. The sand, when pushed back will help to keep the background down, together witth the rockwork that I'll replace against the background.
How can you be sure that the sand will hold it in place even if the top leans on a glass strip?? I have the same situation and I'm planning of draining the tank etc because I read stories when the background just sky rocketed out of the tank
A lot depends on the kind of background you have and the kind of tank you have. Some store-bought backgrounds can be installed in an established tank, see here:http://www.designsbynature.net/2010/08/17/installing-a-slim-line-background-to-the-back-glass-of-tank/
An all styrofoam DIY background will be extremely buoyant, and I wouldn't recommend trying to put one into a tank without taking a few days to silicone it to the back of the tank (with no fish inside).
I made a rock and styrofoam background and installed it into an established tank. I filled several coolers with tank water, took out all the decor, put all the fish in the coolers (I was also buffing the outside of the tank didn't want to freak them out too much), and siphoned out some of the sand (I knew I'd be stirring it up, wanted to be sure it was clean). I moved sand away from the back, and set the four sections in place. I added more sand and rocks back. At first the pieces didn't want to sit flat against the back, but then with the tank full they floated, and in my acrylic tank, I could wedge them under the top and into place. In a glass tank, I would recommend bigger rocks and less styrofoam, so the pieces don't float. I used drylok to paint the background, so it didn't need to cure in the water, just had to let it dry a few days first. I would not rely on sand to keep a background in place--in just one day my fish have dug away a lot of the sand I put against my background.
Well my tank is 120cm (47") long and there is one brace from each side. My initial thought was to take out half the water, take out all the rocks (won;t be using those rocks anyway) and push the sand to the front of the tank. I would then place the glass-fiber reinforced plastic background and silicone it under the back brace (the top part is always dry). I have a 7cm (2-3") gap from the bottom to the lower line of the background so I would place two big rocks lying flat on the bottom and wedge the background between the rocks and back glass. I would then cover the rocks with sand. I thought that this would work but as I said above I read these stories and convinced myself to take another approach ( http://www.cichlid-forum.com/phpBB/viewtopic.php?t=223820&highlight= )...