Rain water won't have any calcium in it. Pollutants are a different story.... How often do you have to top off your pool? Remember, the only way calcium is going to leave your pool is by splash out, back washing or dilution so, if your tap water test 500, I could easily believe you have a reading of 1500 however I have never heard of a CH reading that high before. Around here, you see 250 at the most and I have seen pools in the 800 range. Maybe another Floridian like Evan can chime in and give some good recommendations for dealing with hard fill water.

Dave