Sheikh as-Si‘di رحمه الله said: “The correct is that water is of 2 types: طهور مطهر، ونجس منجس“ Why did he say طَهور مطهر if tahoor already means purifying (it includes purity in itself & purifies others)? Does the meaning of طهور change when next to مطهر and means now طاهر في نفسه only ?

Ṭahūr means pure in itself and purifying to others, as Shaykh al-Fawzan defined it, and the fuqahāʾ explain ṭahūr as muṭahhir (purifying). Imam Ibn Baz used the same pairing for sea water, calling it ṭahūr muṭahhir from impurities and from ritual impurity, which is the full purifying meaning.