dark splotches on my sand

[reefer newby]

Another pic

 

[Tahoe61]

Cyanobacteria, in areas of lower flow?

 

[spspirate]

Thats what it looks like to me too. I have a wooden dowel that I got from Low's that I use to chop up the sand from time to time. Corals love it because releases some detritus into the water column.