O.K. So here's the skinny. Yes, I have a waterfall and I have an auto-fill. I'm running a bucket test as I type and, since 7AM this morning I have lost 2" out of the pool and 7/8" out of the bucket. The pool is 16 x 41 fiberglass and 21,000 gallons. I have no idea what the surface area is (nor does the manufacturer - I called) so assuming that the surface area is 67% of the 16x41 I lost about 260 gallons in 12 hours. If that's my loss, then I do a complete change every 80 days, which dosn't include evaporation.

I Had the water checked today and my salt reading is right on.

Tomorrow I'm going to re-run the test after turning the waterfall off.

I'll report back.

Ralph