The pair had secured podium finishes in the first three legs in Beijing, Dubai and Windsor.
And there were heading for another medal in Russia. They were second going into the final round, but only managed a below par 59.50 points for their last effort. That was more than 20 points down on the same dive in Windsor.
China’s Ruolin Chen and Huixia Liu won in Kazan, with 340.50 points.
Canada’s Roseline Filion and Meaghan Benfeito were second on 304.50 points and Malaysia’s Mun Yee Leong and Pandelela Rinong Pamg took bronze with a score of 303.42.
Couch and Leeds’ Toulson finished with 294.30.