Ok ... I have read lots of articles as well as viewing on youtube the process of creating rock backgrounds uising styrofoam and concrete. In all cases, it is suggested that the concrete can be colored if one desires it. But no where do they say exactly what can be used to color the concrete. After additional research, I have found several methods for coloring concrete. Some indicate mixing the color (either dry or liquid pigments) in with the concrete when it is being mixed. Most others indicate a process for coloring after the concrete drys. Those methods talk of using one of the following: acid stains, acrylic stains, dyes (such as Rit ). There is even methods for Faux finishing concrete with these stains. They all look interesting, but which ones are safe in the aquarium??? So my question is this ... is there anyone outthere who has SUCCESSFULLY collored a concrete background? If so what method did you use? Thanks for your help!