If i'm not mistaken, this is wrong. If you change 5 gallons of old water, then you change out a (in this case a 40g tank) 1/8 of all bad stuff. Then next day you change out 5g, then you're taking out 1/8 of the 7/8 that remains except it's diluted further, so you could be taking even less! So for most efficient water changes, lots at once is better. However, you have to look at it from the fish point of view. I think if you broke it down to a two water changes in a week instead of one. This will make the params shift less, but you have your hands in the tank twice instead of once.. which is more stressful? Hands in the tank a few extra times or param changes. If your doing you water changes correctly, the params shouldn't change at all except for taking bad stuff out. Just my two cents. Evan.