https://aluminumstock.com/why-put-aluminum-foil-balls-in-toilet-tank/
Placing aluminum foil balls in toilet tank is considered a water-saving method. Theoretically, when the aluminum foil ball is placed in the water tank, it will increase the volume of the water tank, causing the amount of water in the water tank to decrease. This can reduce the amount of water used during each flush and achieve water conservation. However, the water-saving effect of aluminum foil balls in toilet tanks is not significant, and not all toilets are suitable. The design and operation of toilets vary by brand and model, and some toilets may not be affected by foil balls because they have a specific flushing mechanism that does not rely on tank capacity.

Additionally, foil balls may cause other problems in the tank. It may interfere with the mechanism inside the tank or affect the normal operation of the tank. In addition, the presence of aluminum foil balls may hinder the flow of water inside the water tank, leading to problems such as incomplete flushing or odor.