Is adding pond salt to tap water necessary for plants? will the amount needed, if needed, be effected by the minerals added to the water through the addition of rocks and pebbles that contain silica?

Plants don't like salt so they don't need any added to their water. Unless the minerals in the water include sodium or sodium chloride, they will not effect the common salt levels. Other salts may be present that provide some of the same benefits of regular salt. Silica is basically inert sand and does not effect salt.