Live rock is a great way to quickly bring a new reef tank online since it typically comes teaming with life, including beneficial nitrifying bacteria. This type of bacteria is vital and provides natural biological filtration, helping to break down organic waste from critters such as fish.
The key to a very quick cycle with live rock is to minimize the time it is out of water since any dead or dying matter on live rock can release ammonia during the decaying process, which is very toxic to fish, corals and other organisms. So a good population of beneficial bacteria are needed to help neutralize and eliminate the ammonia. One species of beneficial bacteria break the ammonia down into nitrite while another species converts the nitrite into MORE