Hard water concentration is different everywhere. In some city water, the 8% cross-link resin works best. The “cross-link” refers to the bonds inside the resin bead. The 10% resin has more bonds that hold the beads together so that they will last longer. Over time, resin beads will fracture and break down into a mush-like paste. 10% one is premium grade resin and is used in very hard water areas.