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Step 3: Lay the first layer and pieces

Picture of Lay the first layer and pieces
Since I decided to do just one side and the back I measured the back first. I wanted the background to come to within 1 inch of the side I was not going to cover, this was because I wanted to taper the edge off and not have a sharp square end showing. I then measure the left side of the aquarium, I decided on this side I was only going to cover about 3/4 of it. Then cut your foam to fit. My 75 gal aquarium was 4 foot long on the back and I needed about 10 inches on the side. I measured 1 inch shorter because of the taper effect or the back piece. I also had to make my background in 2 pieces to get it inside and past the center support when I am finished.  I also cut the pieces 2 inches shorter so that there will be sand under the edge when I  am done. I started with the back corner. I first used the silicone and glued the two corner pieces together along with the first bottom "rock" cut out of the foam.  I used the finishing nails to hold it together until it dried.  Some people cut the rocks first and test stack them.  I just cut them out of the foam as I needed them.  I just used my imagination on the shape of the rocks.  This is where a foam cutter comes in handy.  If you dont want to make one or have one, just use an electric knife or even just a normal knife and cut and carve.  Remember rocks are not perfect but also remember small details will not show up after you coat the thing in cement.