I am actually indifferent to what percentage it is also.
To Mr. X: I questioned another poster's reply that provided no basis whatsoever for his or her claim. Your reply started with the word Because which led
me to understand that you were trying to explain the same exact point I was questioning.
To The Todd: I also don't quiet understand a claim that states "10% a week is nowhere near 40% a month" without any explanation which in my view could only be mathematical as well as chemical to some degree. I understand you said "some newer and older water" in your reference to dilution. But as a mathematically challenged person I don't find it persuasive. Sorry.
I don't really have a problem with either one of you personally as both of you have contributed to my earlier threads in a positive way. I just wish there were more data in your answers to support your claims.
I can explain my statement by the following reasoning which is by no means perfect mathematically as dilution is definitely an important part of the process. When I do a 10% water change on 10 gallons, I am putting 4 gallons of new water per month which constitutes a 40% change.
But like TheTodd said "water changes are important, as they both replenish nutrients and extract nitrates."