Dan,
It sounds like you have fill water that is high in Calcium Hardness (CH) and that you have a lot of evaporation and refill which increases that CH but don't have a lot of dilution from rain overflow or from backwashing (perhaps a cartridge filter?). Do I have that right?
You did not write about your other water chemistry parameters such as pH, Total Alkalinity (TA), Cyanuric Acid (CYA) and usual water temperature. It would be helpful to know those number as well.
Your water clearly has a high saturation index (which you can calculate using The Pool Calculator) so is producing scale. My concern is that this scale may not only be at the tile line, but also in your gas heater (if you have one) and even in your pipes, filter, pump and other components.
Have you been using any kind of calcium metal sequestrant or anti-scaling product? I think it would have been wise to be diluting your water some every year to try and keep the CH to a more reasonable level to prevent scaling. You may also consider getting a pool cover to prevent evaporation as that, plus subsequent refill, is likely the primary cause of the rising CH. If you didn't want to heat the pool too much (say, in the desert or other hot environment), you could use an opaque white or reflective cover instead of a standard blue or clear bubble-type cover.
If you do a water replacement, be careful of your water table and the kind of pool you have (plaster, fiberglass, vinyl; in-ground, above-ground) as you can damage the pool from a full drain. If this is your situation, you can either do multiple partial drain/refill or can use the large sheet or multiple silage bag method for water replacement.
Richard
Bookmarks