What did Elisha do to heal the bad water at Jericho?

Answer: Threw salt into it

Elisha threw salt into the spring and declared the water healed.

“and Elisha went out to the spring, cast the salt into it, and said, “This is what the LORD says: ‘I have healed this water. No longer will it cause death or unfruitfulness.’””— 2 Kings 2:21 (BSB)