I'm confused by something regarding water changes. Let's say to change 25% of your 80 gal system, you mix 20 gals of water up and complete the task Monday night. You've decreased the level of everything in your water by 25%. Let's now say instead you change 5 gallons each night starting Monday, and don't finish the task until Thursday. Is there a simple mathematical way to express the difference between changing 25% of your water both by a "one shot" representation, and again by a 5% per night over 4 nights method? In my mind, by doing it over 4 sessions, you're changing newly changed water Tuesday, Wed's and Thursday; and would hence be less ideal--but I could be wrong. Could anyone help me out here?