I have massive problems with this. When i do a water change i lose at least 2 fish (tetras and raspboras). My tank water is 6.8-7.0 and my change water is pH 8-8.2. Soon as i do a WC, my fish just dont look happy. They get red streaks thru thier fins and yeah, some die. I could not for the longest time figure out what was killing them, but according to others on varying fish forums, they definately say its the huge pH swing, even if i do only a 20% WC, i still lose fish. Im now trying to lower the change waters pH before adding it to see if that makes my fishies live longer. See with pH, every 0.1 change in pH is 10 times +/- the previous pH. Obviously others dont have problems with the pH swings, but i do for sure. My tank is only a 10gal tho, whether that makes a difference, 50% WC is still a 50% WC no matter the size.