What soluble compound of calcium or magnesium might be in water? The hydroxide is somewhat soluble. The carbonate is formed in caves when water containing some of the hydroxide comes in contact with CO2 in the atmosphere. Over years, stalactites and stalagmites are formed out of limestone. When you mix carbonate developers with hard water, you get cloudy developer from the calcium and magnesium hydroxides. You will see it very soon after mixing. Many commercial developers incorporate a chelating agent to keep the limestone from precipitating. If you mix a little sodium carbonate with your water, you will see the cloudiness form. It is true that the sulfites are also insoluble, so you should also get a precipitate by adding a little sodium sulfite to the well water.
I have mixed developers in my hard water with no problems except the cloudiness, and I have not seen that to cause any processing problems. If I must use carbonate, I prefer to use a chelating agent with it, but most of my developers work with borax, Kodalk, TEA or even sodium hydroxide, none of which cause the calcium to precipitate.
I do not use photo flo. I have done, but no longer do. It does just as well to wipe the excess water from the negatives with a paper towel or a well-used and soft cotton towel. Baby's diapers would be good if I could still get them. If you tell me I will ruin my negatives, I will tell you I have never ruined any by this treatment. It has been years since I ran into an emulsion that was soft enough to be scratched by a soft towel. It has also been years since I ran into any kind of water marks on my negatives. I also dry them with a small electric hair dryer. Does this confession mean I will be excommunicated?