Why do I still have mineral buildup?

[Ray]

It could be manufacturing residues in the LECA.

• The starting clay is not pure.
• Then add chemicals to bind the particles together and others to literally take up space in the pellets to create pores.
• Fire the pellets.

The additives and impurities in the clay - both organic and inorganic - can react with the additives and form soluble salts that reside throughout the pellet. As you water, they dissolve, are transported upward, and deposited when the water evaporates.