I'm still trying to figure it out, I did a 20G water change and still coming back on the same spot I even vacummed that area, my sailfin tang is eating it? what do you guys think? nitrates 0. I do feed daily frozen food, 2 cubes for my cowfish and everyone else. formula one, two, mysis, brine. some times flakes but not often. My cowfish won't eat anything but frozen food.