Mineral system (copper ionizer)
Also called: mineral pool, copper ionizer, copper-silver ionizer, mineral cartridge.
A mineral system or ionizer (Nature2, Pool Frog, electronic copper-silver ionizers) releases copper, and sometimes silver, into pool water. Copper is a real algaecide, but copper is not a sanitizer, so a mineral pool still needs chlorine. It just runs a lower level.
Why it matters
Copper suppresses green algae at roughly 0.2–0.5 ppm, which is how these systems cut chlorine use. The cost is what copper does to the pool. Above about 0.3 ppm, and especially as pH drifts up, copper stains plaster and grout blue-green and turns blonde hair green. Copper does not break down or filter out. A metal sequestrant only holds it in solution, and the real way out is to drain and refill. The ideal copper level is 0 ppm. Free chlorine held at the target for your CYA prevents algae with nothing to stain.
In numbers
EPA approves copper-ion devices at about 0.5–0.6 ppm free chlorine, not zero. A chlorine-free mineral pool is the same overselling as a chlorine-free salt pool.