Hi All ... I saw the video and have read several threads on many sites relative to the making of rock backgrounds utilizing concrete. They all mention the ability of the backgrounds to be colored. However they fail to mention what medium of color they use. I researched "coloring concrete" and have found several methods. There are dry pigments, liquid ironoxide pigments .... there are acid stains and acrylic stains and also dyes. Additionallu It seems that there are a variety of thechiques ranging from colors applied in the mixing process to techniques for applying to previously set concrete. The real question is which of these is safe for use in a tropical fish aquarium? Does anyone out there have any experience in this area?