The supplies you'll need are styrofoam, knives, a blow dryer or heat gun, Quikrete Mason Mix Cement, Quikrete Concrete Acrylic Fortifier, cheap 2" paint brush, 100% silicone caulking, and Concrete paint (optional).

The first step is to cut the styrofoam to the size that will cover the back of the aquarium. I put tape down on the garage floor to the dimensions of the tank so I could set up the background there. Plan for the styrofoam to be in 3 pieces. If you don't, the complete structure won't fit into the aquarium (for larger aquariums) because of the cross bar in the middle. It helps if you cut these 3 pieces in a puzzle-like fashion so they'll lock together in the tank. They can be cemented together at that time with the 100% silicone caulking.

Next, using silicone caulking, glue smaller, miscellaneous pieces of styrofoam onto the larger pieces so that you have 3 large sheets with 1-2 layers of smaller pieces on top. Next, you'll need to cut holes in the styrofoam in order to make it appear like a rock formation. You could even create some caves and tunnels if you wish (this is where a smaller knife comes in handy). I used an electric kitchen knife to cut the styrofoam pieces to size and desired shape. Make sure you allow places for filtration and other tank accessories when designing your background.

Once you have your basic shape cut and crafted, you can heat the surface using either the heat gun or blow dryer in order to smooth out the styrofoam's sharp corners. The knifes can also be used for this while cutting the styrofoam pieces to size. This and all subsequent steps should be done outside or in another suitable, well-ventilated area.

Once you are happy with the three individual styrofoam pieces, it's time for the cement. I cemented my three styrofoam pieces outside of the fish tank completely. You could install your styrofoam now and do the following steps with the tank laying on it's back. I followed the instructions for mixing on the cement bag, but then added the Acrylic Fortifier per the directions, and then added additional water until the cement was very runny. I applied this layer using the paint brush to ensure a complete layer over the styrofoam. This layer's function is to bond with the styrofoam and provide a surface for the remaining cement to bond to. Coat each of the three large pieces and then allow it to cure for at least 24 hours before continuing. I coated the back surface of my background with this layer, but it is not necessary.

The second layer is where you will get the bulk of your cement forming and futher development of the rocky look. Mix the cement following the mixing instructions on the cement bag, and then add some more cement powder until the cement will hold it's shape. Then, using your hands, apply the cement all over your background until it has your desired shape Once complete, allow this layer to cure for at least 48 hours.

The final layer provides the finishing touches. If you want to add concrete paint, this is the time to do it. Mix the cement so it is runny and can be applied with a paint brush. Then, coat the entire background with this layer. While doing this, you need to decide if you want a smooth look, or a rough texture. You may need to adjust the mixture in order to get the right look. I went with the rough texture, so I left my final coat runny, and applied it with a brush over the rougher second layer. If you wanted a smoother texture, you would need to make it a little less runny, and apply it with hands or brush to get the right look, filling in the gaps on the second layer. Once complete, allow this layer to cure for at least 48 hours.

Follow these next guidelines regardless of when you're putting the background into the tank. You need to coat the back of the styrofoam heavily with the silicon. Use it all up. The bouyancy of the styrofoam needs a lot of silicon to hold it down. Obviously, add one piece at a time, apply the silicon right before putting the piece in the tank. It helps to set the pieces up outside of the tank to make sure you have everything right. Once all pieces are in the tank, you need to allow at least 24 hours for the silicon to cure. To be safe, wait the time specified for a total cure on the silicon itsself. It must be totally cured before adding water, otherwise the silicon may give way.

Once you've added water, setup a pump to circulate the water. Check the pH after the first day, it should be elevated. Perform 50% water changes every 2 days or so for at least 2 weeks. If the pH was elevated, wait until it is consistently back at your normal level. In my opinion, it's better to be safe. My tank actually had water in it for a month before I added my first fish.

At this point, your tank is ready to go! Your fish will enjoy the caves and other features you've added for them.

Note: In building this tank, I also used the undergravel jets discussed here. □