Soaking a Shower Head will do very little since you cannot see inside and confirm clean and know when it becomes toxic again. If you cannot see inside, there is no guarantee. In 1981, the Annals of Internal Medicine published a study where a Hospital did just that with the Legionella Bacteria in a Hospital Shower Head. Only they used Ethylene Oxide(Poison). The Legionella re-populated in the Shower Head.