Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
1) The nitrifying bacteria seem to handle the changing GH and KH just fine. They thrive in hard, alkaline water, though, so making things too soft might slow them down. Monitor the conditions while you implement step 4 and see how the bacteria is doing.
2) Plants seem to adapt pretty well. I think Crypts melt if you change them around like this, but they should recover. Look into Alum treatment for snails.
3,4) The fish will not adapt very well to a sudden change in water hardness.
I would start now, and with each water change blend just enough distilled water so that the GH and KH drop by 1-2 German degrees of hardness max. (1 degree = 17.9 ppm). A few days later do this again. Do not just suddenly make the whole change in water hardness in one go.