I believe I read once (RC?) that your tank fares better to do a larger change because it removes more of the excess at once, kinda like only cleaning a 1/4 of you filter sock versus all of it. Although, it does seem to make sense to change it as needed based on the need of the tank at any time. No gospel from myself however. I change out 20 gallons of my 90 gallons total every three to four weeks, probably would be better to do it twice a month though.