Mineral Mud rocks. But I think just sand is fine. The calerpa grows like crazy with mineral mud. Sometimes I think it grows too fast.
I think skimmerless tanks are fine. I never really had any major algae problems like alot of people do except for the inital cycle period when I got cyano but that went away pretty quickly. I never had any hair algae problem nor any bubble algae problem. I have this brown fuzzy algae right now but I only had like 2 snails in my tank. Now that I've added more snails, the brown fuzzy stuff is no problem either.
I am not saying skimmers are useless. Skimmers are very good indeed! After I added a skimmer, water quality seemed to be even better. The water was noticably clearer and my sps seems to color up better.
I'm using RODI water also so that contributes to my virtually algaeless tnak.
