Why do I still have mineral buildup?

[Reeses]

I recently made the conversion to using R/O water for watering. At my old house the water was extremely hard, and left mineral buildup very quickly. My hope was that using R/O water would negate (or at least minimize) this problem.

I have a little paph seedling (it said it was "near blooming size") in S/H. When I saw it was getting the buildup even with R/O water, I repotted it with brand new LECA. But the mineral/salt buildup came back! How come I'm still getting this buildup? I'm concerned it might damage the roots on this little paph.