I have a refiltering system. It takes 5 gallons of displaced (by auto water change) tank water and refilters it first through a 100 micron sock canister then a 5 micron sock canister then nukes it with ozone driven by a air stone then goes trough two carbon cartriges. Then the 5g + 5g of new get used for another water change. So one day it gets 5g of fresh and the next it get 10g of half and half.
Don