وَلَا بِمَاءٍ غَلَبَ عَلَيْهِ غَيْرُهُ
Meaning: Nor is purification permissible with water over which another substance has become dominant.
Dominance refers to the point where the additive, rather than the original water, determines the character of the mixture.
A slight admixture and one that transforms the substance are not treated identically.