I bought a new home and during a minor toilet repair I found something in the toilet tank. It's about 5 inches long by 1 inch wide by 1/2 inch thick. It appears to be the same color as a magnet and is very weakly magnetized. It is inside a clear plastic holder that has snap on ends. The holder has four 1/8 inch holes that allow the holder to fill with water.
I only found one in the main bathroom toilet tank, not in the other two.
Any one able to tell me what it is.

I only found one in the main bathroom toilet tank, not in the other two.
Any one able to tell me what it is.
