Conch is considered as a very auspicious thing. It is used in many rituals like while offering water to Gods and Goddesses. But, one should never offer water in a conch to Lord Shiva.
According to Shivpuran, there once lived a demon king Shankchud. He was the son of demon emperor Dambh. Dambh has no children so he performs penance to appease Lord Vishnu.
Pleased by his devotion, Lord Vishnu grants him his wish of having an invincible son. Soon, Dambh has a son whom he names Shankchud.
Shankchud also performs penance to please Lord Brahma. When Lord Brahma asks him his wish, Shankchud asks for powers that no God can kill him.
Brahma also asks him to marry Tulsi. Her virtue made Shankchud even more powerful.
Shankchud attacks the demigods and defeats them so they seek help from Lord Vishnu. But, Lord Vishnu expresses inability to kill the person who is born because of his boon. So Gods seek Lord Shiva’s help, who agrees to help them.
Shankchud drew strength from Tulsi’s purity and chastity. So firm was her chastity that even Lord Shiva could not defeat Shankchud in battle and all the demigods went to Lord Vishnu for help.
So, Lord Vishnu went to Tulsi disguised as her husband, Shankchud. When Tulsi greeted him, thinking that he is her husband, her chastity was momentarily broken. Taking advantage of this Lord Shiva kills Jalandara.
When Tulsi understood what happened, Lord Vishnu revealed his original form. An enraged Tulsi cursed Lord Vishnu for his stone hearted behavior to become a stone. Honoring His pure devotee, Lord Vishnu accepted this curse and promised to appear as the Saligrama shila in the Gandika river (now in Nepal).
He also gave Tulsi devi the benediction that she will eternally reside with Him as His consort in Vaikuntha. He said that for the benefit of every one she will assume the form of a plant which will be most auspicious for anyone performing devotional service.
Shankchud’s ashes turns into a conch, since, he was a devotee of Lord Vishnu it is believed that offering water to Vishnu with a conch appease him. But, as Lord Shiva killed Shankchud it is believed that he should not be offered water in a conch.