Vinegar

Vinegar was a human-made preservative, used for pickling and flavouring foods. Vinegar contained acetic acid, which could kill Raxacoricofallapatorians because of their calcium-based biology. (TV: World War Three, TV: Revenge of the Slitheen)

Mickey Smith had several vinegar based foods including pickles, pickled onions, and pickled eggs in his pantry. Jackie Tyler used these in order to kill the Slitheen that had gotten into the house. (TV: World War Three)