Captain Planet and the Planeteers (1990)

Season 2: Episode 10

Sludge and four of the Planeteers learn first hand the terrible effects of untreated sewage when MAL and Dr. Blight shrink the EcoSub (and them) to microscopic size and put them into polluted water that Kwame unwittingly drinks. The Planeteers must battle hideous water-borne parasites in Kwame's body so that he can recover, and restore them to proper size.