Okay, because it's summer I've been a little lax about testing my spa water daily, but I didn't expect this!
Temp 100° (that's ok, I turned it down last week)
CH 0 (the test water didn't change color at all when I added drops)
TA 0 (the test water turned bright red when I added drops)
I did have a small floater with bromine tabs in the whole time, and the spa has been covered. The floater had two out of its three slots lpen to release bromine.
I think I'll do a complete water replacement today anyway, but I'd love to know if anyone has an idea what could have gone so wrong with the water in three weeks!