yes both are bio filtration but the bio balls do not break down nitrate as well as live rock and the bio balls also build up bad bacteria with the good. live rock will also spawn pods in the sump. that will not get eaten by the fish which will allow more to grow and you -could switch out the rock for a treat for the fish. also micro algea will do good with the rock in th sump. its a better more natrual enviroment.Kullas said:what would be the differance in taking the bio-balls out and replacing with live rock and just leaving the bio-balls in. both are used for bio filtration?