I use powdered citric acid on pests like aiptasias and green algae. It's like 55 cents an ounce in bulk at Aquavita. The aiptasias melt like slugs sprinkled with salt. I suppose the citric acid does the same as kalkpaste but on the opposite pH range? This stuff is pretty strong, and may eat through rocks if left on for too long. I haven't done any research on using it, but it cleans my dirty rocks like crazy, and literally melts the aiptasias.
I should add that I take my rocks out as I treat with the citric acid. No clue on what that stuff would do to your water column if you can't take the shroom rock out...