[motorslave]

The live part of live rock is the bacteria and they are the filter for the tank. They process waste. You dont need it but it sure makes for a healthier tank. If you buy dry or base rock it will, over time, become live as will any rock you place in the tank. Bacteria colonize the various surfaces in the tank.

 

[motorslave]

without live rock you would need a filter or skimmer!

You will still need some other form of filtration such as a canister or a skimmer. But, you can get away with a bit more in a fish only system.