Just an FYI, I started with some of the more dense rock. Not all of it, but I had 2-3 bigger pieces in my tank and my sump was full of it instead of bio balls. It became one of those things that just stayed in my mind and bothered me quite a bit. After looking at it more thoroughly, I decided it was barely porous at all and have since replaced ALL of it. Again, maybe it was just me, I just feel better about not having it and knowing I have better quality rock that will be more beneficial for my tank as a whole.