Soap

From The Aquarium Wiki

Soap is never used in the Aquarium hobby unless by the most experienced aquarist.

Soap whilst being a common cleaning material in the home, it is highly toxic to all forms of aquatic life. The smallest amount in the relative small volume of all tanks 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 repeatingly rinse your hands extremely thoroughly and dry them with a clean cloth.