Hi I want to start my build but I'm having trouble figuring out what to do about my walls. First off my area is 12'Wx7'Hx6'D it will have a pre-molded 4'x5.5' pond with heater and pump lots of live potted plants and plenty of light. I was thinking of lining the whole thing with high density foam board 1'' thick to keep in heat as it gets cold hear in the winter. I need some ideas as how to make light weight artificial rocks or something to do my walls with. Grout or concrete would be far to heavy and also keep in mind non toxic and able to stand 60-80% humidity over a long period of time any ideas?