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.