It's pool filter sand, got it at a local pool store. It was 7 or 8 bucks for a 50 pound bag. I went with 50 lbs at first, but decided it wasn't quite deep enough so I put another 20 or so in.
Tank looks very nice, as do your fish. I agree with Ib3ak on the ID. You do need a heckuva lot more rockwork in there.
Sorry about your venustus - I would have been advising you that they can get too large for your tank anyway, and need more swim space, especially when mixed with Mbuna. As you ask about stocking, I'd be worried about your auratus and your crabros. The auratus can be true killers and the crabros are, frankly, jerks and because of their size can really bully other fish. Both fish get distinctly unattractive, in my opinion, when older. I presume you havn't stocked all male, but have 1 male to each 2 or 3 females, which is the ratio in which almost all mbuna should be stocked unlesss keeping a single male specimen.
You should identify the pleco - post a pic here, or try www.planetcatfish.com for help.
Here's a couple crappy photos of the pleco (my camera was dead so I had to use my phone)
He's been acting pretty crazy lately. He's obsessed with the bubble curtain. He swims up and down constantly through the bubbles. I watched him do it for about 15 minutes yesterday. Crazy fish!