Quote Originally Posted by CarlD View Post
I'm guessing that this is actually creating what is called in household water, hard water, which is why water softeners are popular (we have one). Hard water is notorious for being difficult to form suds in, for breaking it down quickly, and, of course, for leaving scale on everything. So it makes sense that if you make the water harder, foaming/sudsing will be more difficult. I realize this is different in that hard water is usually other metals, particularly iron and copper, but calcium is, of course, a highly reactive metal, but a metal nonetheless.
Hard water is usually from magnesium and calcium, not from iron or other metals that aren't very high in concentration. Magnesium and calcium form a precipitate with soap such as magnesium or calcium stearate. This is why soap doesn't suds well in hard water and how soap scum forms. To reduce foaming in spas, one could use magnesium instead of calcium if one wanted to.