A dog walker was alarmed after she and her pooch discovered fluorescent green water in the canal at Little Tring.
The quick-thinking woman, who did not wish to be named, posted the photo to Facebook’s Everything Tring page where commenters expressed their concern for the fish and surrounding wildlife.
But the brightly-coloured water was the result of a harmless powder called fluorescein, which is being used by the Canal and River Trust to check for leaks.
Principal waterway engineer for the Canal & River Trust Neil Owen said: “We recently carried out some dye testing along the Wendover Arm to check if there was a small leak from the canal into a neighbouring property.
“Bright green dye was used as it clearly shows us where the water is flowing, and if the reported leak was coming from the nearby canal.
“Although its looks really strange seeing the canal a bright green colour, the dye is completely harmless to wildlife and fish and will naturally disperse after a few days.”
Wendover Arm Trust’s restoration director Roger Leishman said: “I used fluorescein myself when I was a civil engineer. It helps you see the water down a dark manhole!”