Brand new containers when I started this a few weeks ago, and haven't had to refill yet. Mixed each part in it's own container. New dosing pump and tubes. I mixed everything seperately. I would concede to user error, somehow, but I just mixed up another batch of the BRS pre-portioned mixes this weekend in new containers, and put them in new VOSS dosing containers I made, for my coral frag/QT tank, and I'm seeing the same sand sized particles in it. This tank doesn't have a sand bed, and never has so it's not sand. I didn't mix the two parts.