i use 200, 100, and 50 micro socks.. i think once tank ran through with 100m for the 2-3weeks and the 100m is dirty i would change to 50m felt and that last me 2-3 weeks... i never stacking my socks up.. once that dirties up i would clean the sump up and start all over with maybe 200 all the way down to 50.. easy to keep track of cleaning sump and media in the sump... from initial 200m down to my 50s i have about 3-4 months...