Now when you mention this there was a time when I always changed my water with one hose (siphon) coming out of the tank and one with fresh water into the tank - without altering the water level.

No pearling. We have < 0 ppm CO2 in our tap due to hydroxide treatment, and very low nutrientlevels.

Since I started to do ordinaray wc:s where I lower the surface to just a couple of cm from the substrate the "after wc champange" have started again... Hmm...