Guppies (at least, wild ones) can tolerate 1.5x sea water, if acclimated. So if you had guppies die, I don't think it was the salt water.
Can you describe your tank more?
What kind of filter, what kind of gravel, what kind of aeration? any other rocks or ornaments?
Can you describe your tank more?
What kind of filter, what kind of gravel, what kind of aeration? any other rocks or ornaments?