Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
As noted above, this substrate will keep on adding ammonia to your water for at least a month. Do not put fish (especially growing fry) in there until the bacteria population has grown to the point that both ammonia and nitrite test 0 ppm. The soil may still be producing ammonia, but then there are enough bacteria to keep it under control.
Yes, this substrate will remove KH, down to 0. This allows the pH to drop to the low 6s.
The bacteria that you want to grow do not grow well under these conditions. They need the minerals that this substrate is removing from the water. Add baking soda or other source of carbonates to keep the bacteria growing. Once the bacteria are well grown, and the plants are well established, then it is OK to allow the pH to drop. The bacteria may not grow and reproduce so fast, but they won't totally die out. The plants are part of the bio-filter, too, and they keep on removing nitrogen in all the common forms.