Start of foam carving and gluing on more pieces.
Rough outline of rocks done. You can see some black lines between the rocks. That's where I cut it into pieces to be able to insert it into the tank.
First fitting. All went well except I had left too much room between weirs and foam.
More foam and carving.
Much better fit. At this stage I went over all the rough edges of the foam with a mini blowtorch. This part smelly so I had to make sure not to inhale any fumes.
First of two layers of cement.
Took 6 1/2 bottles of silicone to glue all 4 pieces of background in and seal around the edges.
Only had black silicone left and was impatient so used it to seal between each piece. Once dry I covered all seams with more cement.