My system went online 05/17/2010 & I have NEVER done a water change.. So, that is well over a year now. I loose 3gal of water a day from evaporation, that my ATO system takes care of. If that is the case, 21gal of fresh RODI water goes into my system every week. That is 84gal a month of new water going into my system. The salt never leaves the system. My clowns, blue-green chromis, & bi-colored angel are over 5 years old & even on my old 29gal I have never done a water change. It also helps that all my skimmers have been over sized. IMO water changes can lead to unwanted swings in Salinity, CA, MG, ALK, etc... Granted if there is a major problem & a water change is needed, it is what it is. But if your system is fine, why mess with it. IBM's golden rule "If it's not broken, don't fix it!"