You can make a removeable concrete background. I have made several and its the only way I'd make them now. Here's what I did:
1) Cut a piece of concrete board (DuRock or similar) to the size you need. Consider if you need your background in sections to install in the tank.
2) Silicone styrofoam to the concrete board.
3) Carve the background, hollowing out some of the rocks as best you can to fill with concrete. Cover with a couple layers of concrete in the usual manner outside of the tank. Odds are the background will still float. At this point, poke some holes in the concrete and dissolve the styrofoam slowly using acetone in a spray bottle. Pour concrete in the void to replace the styrofoam. Let it dry overnight then check to see if it's heavy enough to sink. I filled a trash can up with water to test the bouyancy. As mentioned above, you may be suprised at how much styrofoam must be removed. The background will be pretty heavy to pick up but if done correctly it will be lighter in the water. When it seems OK, put on your final layer of concrete.
4) Of course it must soak until pH is stable.
5) To install, just put it in the tank. It will sit on the bottom. You will need to make sure the top is held in place. I pushed some foam weather stripping along the sides to wedge it in place and keep fry from getting behind it. Push sand up against the lower edge.
Here are pictures of two of the sinking backgrounds I made. Some of the "rocks" are almost solid concrete. You might also notice that in the first picture, the "rock" in the upper left is hollow and open to the bottom. This is for the filter intake and heater. In the second picture the "tree" is hollow for the same purpose.