I have used it and do still have some, I can't say whether it sped anything up or not... as it was my first time cycling a big tank (well biggest so far anyways - 75g). So i'm not sure how much i'm helping this debate, my LFS (family owned and operated for many years) swears by the stuff. It definitely doesn't hurt anything and wasn't expensive, so whatever can help my fish thrive, i'll try it.