I have softened water and do not have any issues with soap scum, but I do have an issue with biofilm on the sink stopper. I have always assumed that it's related to shaving, since my wife's sink does not have much of an issue with it. Once every week or so I need to thoroughly scrub the stopper (mine has a little basket in the drain to catch anything that might otherwise literally go down the drain) and the drain pipe with a brush and a cleaner. I usually use toilet bowl cleaner for that job and an old toothbrush taped to a chopstick. It doesn't seem to make any difference whether I use tallow or vegan soaps.