Tap = 8.2 or so around here. Knife was in about 8.0 I think due to a air stone falling apart. All of my tanks are a combo of 1/2 tap and 1/2 r/o so my fish don't start getting a water scale build up on them (j/k but this stuff is hard). The tank all the other cats are in is about 6.6-6.8. The current pH in the 600 is about 7.6. the buffer capacity is almost maxed. It's starting to go through the swing. Once I get a little closer, I'll do it.