Nature2 Type M 25 Mineral Cartridge Replacement for In-Ground pools from 5,000 - 25,000 gallons. This is the "M" style for M vessels. The Nature2 mineral technology by Zodiac Pool Systems uses safe, elemental mineral ingredients to create pool water that is noticeably cleaner, clearer, and softer. When do I replace the cartridge? Cartridges for pools last up to six months.