Hello, So I understand the basics of PH and how it plays a role in my tanks but I'm a little confused on something. Why is it that when somebody performs a water change it doesn't effect the PH? Like for example say I do a water change on my tank that is established and has a PH of 7.8 but my tap water is 8.2. So I do a 20% water change but the tank stays at that 7.8 PH. I don't get it..