[mike31154]

Personally I prefer larger water changes on a less frequent basis. I do about 23 gals every 3 weeks on my sumpless 77. Perhaps not as convenient as smaller or automatic changes, but the difference in water quality after the change is immediately apparent. By doing small frequent water changes, I think you end up with more 'old' water in your system long term. I keep track of my water changes on an Excel spreadsheet that I modified from someone else's efforts years ago. It includes a calculation that provides the percentage of old water to new water after each water change. Not sure exactly how the calculation works, my math skills are lacking, but larger water changes are always going to get more 'old' water out of your system than small ones, even if you do frequent small changes. As soon as the new water is mixed in with what remains of the old, your opportunity to further renew with fresh water is pretty much nixed since you will be taking out some of the 'newer' water with 'older' water to make room for the real fresh stuff. Hope that makes sense. Pretty much the reason that small or continuous water changes are not that economical as mentioned by Myka.