I just set up a tank with Black Diamond blasting grit, 20-40 size. I was amazed at how much better it is than the coal slag grit I used several years ago. This stuff isn't at all sharp, is much smaller particles than I had before, and is easy to use. I didn't even wash it. I'm completely sold on it now. It can't harm cories because it isn't sharp edged particles anyway. Now I wonder what brand the stuff I had before was.