https://en.m.wikipedia.org/wiki/Antimicrobial_properties_of_copper

I have a solution. To the bacteria at least (potentially).

Everyone can dump their pennies they can't cash in bulk anymore in, and let what copper that still exists in them do it's work. 

Pennies have a negative value, it is a form of punishment grocery stores use on people who insist on paying with cash. Banks charge you fees for depositing too many pennies in at once.

My idea will not effect the heating effect of the blackness of the balls, or the supposed increase surface area of the water, but hey, I may of finally found a economically viable use for pennies. I think everyone in Los Angeles this year should pay their municipal fees exclusively in pennies until they get enough to pave the bottom of this lake. I'm sure their clerks will welcome the idea.