Rising calcium level

[Lance]

Over the last month my calcium level has slowly risen from 420ppm (NSW) to 480ppm. Although not really concerned yet, I'm wondering what is causing it. Other parameters are pretty good. pH 8.3, S.G. 1.025, 10 dKH, mg 1140ppm, nitrate & phosphate no trace on test kits (Salifert & API). 20% W/C every 2 weeks. I've used I/O salt from day 1. Could it be a pail of salt? Any ideas?