Water collected in Conch or Shankh is not usually poured on
a Shivling while worshipping Shiva. Water from conch is only offered to Vishnu.
As per Shiva Purana, water from conch is not offered to Shiva because he had
killed the demon Shankhchood. It must be noted that this rule of not pouring
water from Conch on Shivling is not strictly followed.
Legend has it that father of Shankhchood was demon Dambh. He
was an ardent devotee of Vishnu and he had no children. He propitiated Vishnu
who blessed him with Shankachood. Thus, both father and son were ardent devotees
of Vishnu.
Demon Shankhchood got the boon from Brahma that he will not be
killed by the Trimurtis – Brahma, Vishnu or Shiva. The boon was linked to the
purity and chastity of Shankchud’s wife Tulsi. The boon will be effective only
till the purity and chastity of Tulsi was maintained.
The boon made Shankhchood arrogant and he started harassing
all living beings.
Finally, Vishnu decided to put an end to the Adharma of the
demon. But Vishnu could not kill the demon as he was an ardent devotee of Him.
Vishnu took the form of Shankachood and violated the chastity
of Tulsi. Shiva used this opportunity and killed demon Shankachood.
As Shankhchood was a devotee of Vishnu, he took the form of
conch. Vishnu gave him the boon that Shankh will be used for offering water to
Him by human beings and Devas. As Shiva killed Shankhchood, water from it is not
offered on Shivling.
Note
– there are numerous versions of the above story. Tulsi the wife of Shankhchood
became the Tulsi plant, which is considered most dear to Vishnu. The demon
Shankhchood is popularly known as Jalandhara. Another story has it that Tulsi
cursed Vishnu to become Shaligram.