07-06-2009, 11:08 PM
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Just a thought.... when you test pH of the tap, you need to allow for excess CO2 to dissolve from the sample prior to testing, otherwise you will get a false low pH reading. For best results, you should allow overnight for this to occur, preferably with the help of an air stone.
On the subject of harness, I agree that you do not necessarily want to adjust hardness. However, with a daily hardness testing you can get an idea of long it takes for the carbonates and other buffering ions in your water to be depleted, at which point the pH will begin to swing. In a freshwater aquarium, testing hardness can be an excellent indicator of when it is time to do the appropriate water change.
For the original question, you stated that your GH is ok, but your KH is extremely low. All this means is that you lack carbonate buffering ions. General hardness is a measure of all the buffering ions, including bicarbonates, borate, and phosphorus. Having a low KH is not necessarily a bad thing, provided your GH is sufficient to prevent pH swings. I would personally add a buffer, such as Proper pH 6.5, to replenish some carbonate ions.