Blackness under sand - Flow problem?

[SF2K4]

Ok, so I have a 20L AIO that I made with a rock wall. The flow is all done via a MJ1200 as the return and then a MP20 blowing from one side to the other. The MP20 blows from the return side back across the tank to the intake side. Anyway, I've noticed that with the sand I bought (CaribSea... forgot the style) had some shells in it, that's okay. However, all the larger pieces have settled in the corners of the tank, especially across from the MP20 near the intake. It's in these areas also that a layer of black is noticeable underneath the sand. I've cranked the MP20 up as high as I can without blowing sand literally everywhere all over the tank. I have some Nassarius snails I thought would sift the sand some but not really to an avail. Anyone help me out on what should I do? thanks guys!

 

[Logzor]

Those are the anaerobic areas in your sand, sounds normal to me.

 

[jefathome]

Yeah, don't sweat it...

If you get some dark RED slimy areas then that is Cyano and is a bit more of an issue.