Thanks. The backgrounds are made from pink styrofoam. The rock shapes are carved from blocks of styrofoam and glued in place using either silicone or gap-filling styrofoam in a can. I glue the styrofoam onto acrylic sheets and then use silicone to attach the acrylic sheets to the walls of the tank (this allows for test fitting prior to committing the background to the tank). The backgrounds can be constructed to conceal the filter hardware.
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I paint the styrofoam using drylok, a latex paint used for waterproofing the foundations of houses. It comes only in gray or white, so I tint it using cement tinting powders. All these materials are available at hardware stores. This technique was something I learned out about from internet searches on DIY backgrounds.
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