Yes, you guessed it correctly on the midwest.
I do get a lot of white residue in the tank, all around the water level line and up above my filter spray bar (from the constant, slight splashing up on the glass above it). Periodically, I take a plastic razor blade, and with a little tank water to loosen it up, it scrapes off easily.
I was doing a 60% water change every week, but then I started traveling a lot with work a couple months ago, and my last 3 changes were done every other week. I will get back to weekly to see if that improves things.
I think my filtration is pretty good. I have a sunsun 404b (rated at 525 gph, I believe) and inside it, I use [from the bottom of the filter, upward] a course filter pad, medium pad, and fine filter pad for mechanical filtration, and then two trays of ceramic rings at the top for bio filtration. I do take care to only use tank water when rinsing the filter pads and rings to preserve the beneficial bacteria.
There is some room for water quality improvement. I will get back to doing a 60% water change each week and will try to reduce the hardness. Oddly (well, oddly to me anyway - maybe it's not really that odd), the pH levels are always around 7.5 - not high at all like I would expect for hard water. I use tetra strips and API drops for testing and they both read the same 7.5 pH. I don't have a hard water test with the API kit, but the tetra strip always shows a "hard" reading, and the visible mineral residue backs it up.
Thanks for the advise and for taking the time to provide it! Much appreciated!