I only use a sock in the outflow from our skimmer. Since it is rated for 800 GPH flow through our tank water gets pretty well polished by the filtersock. By using on the output of the skimmer, the majority of the gunk is exported in the skimmate and we don't have to change our filter sock but every week or so.. sometimes more often, rarely less often. And we clean/reuse our socks, and soaking them overnight in RO/DI has always been a part of our regime .