If your tank is cycled, you can't do too many water changes. Although I do 50% in all but my turtle tank every week, if I did this routine less frequently (like once a month), it could be hard on the fish, because the new water being added might adjust the ph. Since I do it every week, there is probably not much of a fluctuation. More frequent water changes is far more important than how much you change out.Dahly said:This is a good question for me at this time. I have been changing 25% every 4 days, but my lfs said that's too much. I was going to go down to 20% every other week, because of the lfs and so much that I have read. I will stay at 25% every 4 days, and check water chemistry once or twice weekly too. "Freshwater fish like freshwater!" If they were in a stream, the water would be in constant change. I just need to be sure the temp is correct, as not to stress the fish. I see Gunnie and other change their water frequently, so will I! Good poll, thanks!