Concrete/Styrofoam Background - Tips and Suggestions

There are lots of articles and instructions available on how to make this kind of background. I suggest you read as much as you can find on the procedure before you begin. The following is just summary of what I did and a few things that I learned when making mine.

• I used Quikrete Mason Mix along with Quikrete Acrylic Fortifier additive. With no color added, this is a pretty good color match for limestone. Also, it dries pretty slowly, so there's no need to spray it while it dries.
  
  
• The mason mix is pretty much just cement and sand, so no need to add additional sand or anything else, unless you want color.
  
  
• I used 1.5-inch high-density blue styrofoam purchased at Lowe's.
  
  
• For cutting the styrofoam I used small kitchen knives and various size large files. Files work great for rounding edges and roughing up before applying concrete.
  
  
• Try to use the same amount of mason mix, water, and fortifier each time you make up a batch. Different proportions will result in slightly different colors when it dries. This is not really a serious problem, since the background will eventually have algae growing on it.
  
  
• Mix the cement in small batches. I used a Cool Whip container and filled it about three quarters full with each batch of concrete. It took about three of these to cover the entire background for the second and later layers.
  
  
• I cut out a piece of cardboard the exact size of the back of my tank to use as a guide. I laid out my filters, heater, and other equipment on the cardboard and traced them to keep track of where everything went as I was laying out the rock structures. I also traced out how I wanted the rocks to look.
  
  
• I used one solid sheet of styrofoam for the background, cut slightly narrower than the width of the cardboard to allow for the thickness of the concrete on the edges (I didn't want to see blue on the sides of the tank). I cut out holes for the equipment from this big piece, then added additional pieces to cover the holes and to add depth in various places (glued on with silicone).
  
  
• When carving out the styrofoam, don't worry about details because the concrete will fill everything in. Sculpt the details in the concrete, not the styrofoam.
  
  
• If you want your background to match other (real) rocks in your tank, use those rocks as models for the general shapes, sizes, and textures of your background rocks. You can make the surface of the concrete very smooth with a putty knife, or rougher if you go over it lightly with a wet paint brush.
  
  
• Make sure it looks they way you want it to before the concrete dries. Once it's dry you can't change it, only add another layer on top to cover something up.
  
  
• When carving gaps between rocks, make them wider than you want them to end up to allow for the thickness of the concrete.
  
  
• Add depth to the surface - have some areas that stick out further than others, or that stick out at angles. Don't make all the surfaces of the background rocks parallel to the back of the tank. This is a 3D background after all.
  
  
• Avoid symmetry and square, straight edges.
  
  
• After you have everything carved and are ready to apply the concrete, cut the large piece into two or three sections so you can get it into your tank. If you can, divide the piece up between the gaps in the rocks. Put all the pieces in the tank and make sure they fit before you put the concrete on. Be sure you don't put a thick layer of concrete on the edges of the pieces where they fit together. Leave the edges bare where they go together and touch them up after everything is in the tank.
  
  
• Apply the concrete with the styrofoam out of the tank, lying flat. It's difficult to apply concrete on a vertical surface.
  
  
• Rough up the styrofoam and spray it with water before applying the first layer. Spray it again before adding additional layers.
  
  
• Make the first layer thin and brush it on with an old paint brush. This layer will be thin and crumbly, so just leave it sit there and don't move or touch it. Make subsequent layers thick and apply with a putty knife. Let each layer dry at least 24 hours before applying the next. I used three layers over the whole thing and more in some spots.
  
  
• Wear rubber gloves. Concrete is very alkaline and hard on the skin.
  
  
• After it was done and dry, I siliconed the pieces into the tank and then touched up places as necessary, for example between pieces where the styrofoam was showing.

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