While Tom's answer might not be the, uhhh, most concise, it is certainly the most complete. Thanks Tom!!
Having spent a good many years as a health care practitioner, I have some understanding of compartmental dilution between intracelular, extracellular and interstitial components of human fluid and electrolyte balance, and such a phenomena in a captive reef system would certainly explain my observations. I just never thought of it in that context. Glad to know I'm not losing my marbles after all.
The bottom line is I'm doing 20% water changes weekly right now, so maybe I'll be OK in the long run. Or maybe I'll just go back to the drudgery of mixing up and dosing Kalk. Ugh!