I've got a little 3G tank with a half dozen plants in it and one fish for the time being. Flourite substrate, 2W/G, 10mg CO2, pH7.2 and KH currently 7. My water comes out of the tap at pH 7.8 and KH4. I put some baking soda in and raised the KH to 9 but it seems to creep down about 0.5 point per day. Why would the KH drop? Is the substrate stabilizing or something? tnx jtm