I've had plants that grew well for me, and a few that didn't, I've only really ever kept tetras and cichlids and they have lived for years so I think I'm at least doing ok. I recently bough a 75 with the intention of doing the high light, EI, injected CO2 thing. Anyway, I've kept plants and fish for a several years without ever measuring my ph, KH or GH, and lately I've decided to learn a little more about whats "really" happening in my aquarium so I bought a test kit (AP Master Kit with a "High pH" test). To my surprise, my pH was 8.4, kH 9* and GH 21*. I then tested some fresh tapwater and got roughly the same readings (pH 8.4, KH 9* and GH 17*). I have a couple of questions that I hope some of the smart people on this board can answer.

1) I think I understand that kH is mostly a measure of carbonate, and that GH is a measure of calcium and magnesium. With my very high GH does that mean I probably have a have a high amount of magnesium? or could it be calcium in some other form than CaC03? or something else?

2) My pH is pushing the high end of the high test kit so I suspect there may be accuracy issues. It doesn't seem like the GH test has a real upper boundary.

3) The real question, of course, is does it matter? Are there some precautions I should think about?