^^^
Absolutely wrong. I am kind of amazed people are advising this...
It is "dry" but has plenty of dried organic matter upon arrival. I had to remove dried sponges and many other thing that were large enough and washed it many times. Every thing I have seen has said to cycle it like any other rock due to this dried organic matter that WILL DECOMPOSE AND RELEASE AMMONIA.
I have been cycling my marco rubble for 3 weeks in a bucket and it is still reading VERY HIGH ammonia and nitrite, even with weekly 100% water changes
Marcos benefits are the awesome shape, cheap price, and ability to create any structure and take your time drilling it etc...
It DOES NOT have the benefit giving you the ability to add it right to your established tank, as I have come to find out. Please beware. If you want this, get fully cured live rock from your LFS, that is your only choice.
I love the stuff and will use it almost exclusively from now on, but it is hardly "clean rock"... Just imagine live rock left out to dry and everything that will be left behind. The organic matter does not just dissapear, especially when it is coated in a great natural preservative. Get it wet and it will decay rapidly.