lighting; hardness

First, thanks to everyone who responded to our question about Daniel's new
lighting (the lights would only come on if you rubbed the bulbs). He's
added a ground wire to the reflector, which seems to be helping.

I have another (hopefully not stupid) question, this time about water
quality. I've just tested the total and carbonate hardness of my tank water
for the first time, and found that while the total hardness (GH) is
moderately high, the KH is barely detectable! I used those "Aqua-lab"
strips, so I don't think the results are at all quantitative, but I believe
the overall message (high GH, low KH). Should I do anything about the
water, and if so, what?

Other info: I have 2 tanks involved: one 30g with about 1/3 tap & 2/3 RO/DI
water; and a 10g with straight tap water.
   30g: pH 6.9                                                    10g pH 7.5
           GH about 120 ppm                              GH about 250 ppm
           KH below 80 ppm                                KH about 80 ppm
(maybe less)
           angels, tetras, 1 refugee swordtail    livebearers (guppies, a
couple swordtails)

The pH is pretty stable in both tanks (maybe because I sneaked in some
buffer from the lab??) and the fish look healthy. Both tanks also have
medium-density plants, not bad for low or medium light,no CO2, no fancy
substrate, and only occasional iron supplements. (Actually, I have no idea
what I'm doing _right_ or why the plants aren't all dead, but that's a
whole different story!) Thanks for any advice, or even just aimless

   michiko & momotarO's human