Filters and live rocks purpose is to convert waste thru the nitrogen cycle to create nitrates. Nitrates need to be removed by water changes or by plants (anaerobic bacterias too but that is a bit of a complicated subject). Water changes will also replenish needed nutrients in the tank. I like to do 10% changes weekly but it really all depends on your system and its needs. No two tanks are alike even if using the same equipment and same livestock.