foil is used directly on some rusty metal surfaces, gentle rubbing can help remove rust under suitable conditions. In other cleaning applications, aluminum participates in a chemical reaction with other substances.
But placing a ball of aluminum foil in a toilet tank is not the same as scrubbing a rusty surface.
The foil is not being used to physically remove a deposit, and ordinary tank water does not automatically create the conditions needed for an effective rust-removal reaction.
In fact, aluminum can corrode under certain conditions, especially when exposed to
