it is better to leave the tank free of unnecessary objects.
Does aluminum foil remove rust?
Aluminum foil can help remove some rust from certain metal surfaces when used as a physical scrubber under appropriate conditions. That does not mean a ball of foil left in toilet tank water will remove rust or prevent stains.
Can aluminum foil help prevent hard-water stains?
There is no good evidence that putting foil in the tank prevents hard-water deposits. If mineral buildup is a recurring problem, use a suitable cleaner and consider investigating
