I am still going with the high mineral content.
I am a dichlor user, did not use MPS during my first 4 months, and still got those pretty spots. My ph has been sky high and below 7, and I still got those spots.
If you have a water softener hooked up, it should only be hooked up to your cold water--not your hot water and not your hose. You wouldn't want to be drinking, cooking, and watering plants with that "salty" water.
I get a mineral residue ring around my kitchen faucet and I most certainly can tell which pipe is the cold water pipe in my laundry room sink-yuck!
I had asked a question awhile back about if Defender helps with that buildup and never got an answer. I had a tech out who told me that I need to make sure I use it faithfully as he has seen a lot of buildup on spas that haven't used it regularly. I am wondering if what he has seen on the neighborhood's tubs is mineral build-up similar to my faucet, NOT scale formation. I am assuming the hot tub scale is calcium? Yes, and a mineral, but perhaps not all minerals are the same and perhaps one mineral is taken care of with Defender and the others are not. Hmmm. This is an interesting thought. :-?
I live in Limestone bluff country.