Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
There are a couple of ways to lower KH. In turn, this will usually lower the pH.
1) Blend your tap water with reverse osmosis or distilled water in the % by which you want to lower the KH.
If you want KH to be 100ppm (50% of its current value) then blend 50/50 RO + tap. You may then have to correct the GH, if the tap water already has a low GH.
2) Find a media that will remove KH from the water, and treat the water with this. One such material is montmorillonite clay such as Safe-T-Sorb, Turface and related materials. If you can filter the water through this, a lot of the KH will be removed, but IME the GH will stay the same.
I have seen the effect of using STS and other montmorillonite clays as a substrate in aquariums and it removes a lot of the KH. It can also cloud the water, so I would suggest rinsing it first, then filling a canister filter perhaps 2/3 with STS, and the last basket with floss to remove the remaining fines.
Example: in a tank that holds 88 gallons of water, 2" of substrate represents about 10% of the volume, this substrate kept crashing the KH from tap water and from baking soda or potassium bicarbonate for over a month. Every few days I would raise the KH to about 4-5 German degrees of hardness (roughly 70-90ppm) and it would be back down to 0 the next day.
Similarly, a standard 29 gallon tank, 2" of substrate is also about 10% of the volume. In this tank I was not correcting the KH as frequently (only about once a week) but it went on for over a year, dropping to 0 in between additions of baking soda.