The research and things that i've read so far has "Portland Cement Type I/II" ($8.97 @ Home Depot/94lb bag) as the ideal, tank-safe cement. I actually spoken to a guy that has done this and has been quite successful for the past 2+ years. I mean, after all the coraline gets growing, its seriously almost hard to pick out the man made rocks. I have a few pictures that he sent me. I just gotta find it somewhere on my hard drive.