You would have to fill the whole tank with whiffle balls. You'd lose a lot of tank volume, and I doubt all those whiffle balls would improve the taste of your water
. They might also leech all sorts of toxic contaminants over time.
Essentially, the more flexible the plastic, the more likely it is to contain potentially toxic plasticizers called “phthalates,” which make it more pliable. Plastics made of polyethylene terephthalate (PET) or high density polyethylene (HDPE)—such as most Nalgene bottles—are safer than that made of polyvinyl chloride (PVC).