Where to Watch Only Fools and Horses - Mother Nature's Son (1992)

Summary

Strapped for cash as ever, Del is clearing Grandad's allotment, which has become a health hazard and where he finds some bottles of yellow liquid, which he dumps. He also finds a tap, hidden by rocks, but claims that there is an actual spring, yielding 'Peckham Spring Water'. Using a bottle of mineral water for the lab test he gets the water accepted and sold to a supermarket chain. Unfortunately he is unaware that it has been contaminated by the same yellow liquid he chucked in the reservoir and, as he turns out the light, the water starts to glow green.