Soap
From The Aquarium Wiki
Soap is never used in the Aquarium hobby, unless by the most experienced aquarists.
Soap, while being a common cleaning material in the home, it is highly toxic to all forms of aquatic life. The smallest amount in the relatively small volume of aquaria is very capable of destroying bacteria, viruses, fungus and the cells of all aquatic life.
- If you do wash your hands with soap, you will need to repeatedly rinse your hands extremely thoroughly and dry them with a clean cloth.