Rio de Janeiro - Olympics organisers said they would pump millions of litres of green water from a Rio pool after admitting defeat in attempts to get it back to blue.
In a "radical" step, 3.73 million litres (985 000 gallons) will be replaced at the 50m water polo pool, which hosts synchronised swimming from Sunday.
Officials blamed hydrogen peroxide, added to the pool by a contractor, as the cause of the pool turning a murky green.
Clean-up efforts are also failing at the adjacent diving pool, which is a darker green and smelled like a "fart" on Friday, according to one diver.
But while the diving pool can stay green without affecting competition, venue manager Gustavo Nascimento said synchronised swimming requires crystal-clear water.
"We've been trying for four days. Obviously it's not going as fast as we wanted... so we're going to change the water," Nascimento told a press conference.
"We're going to drain the water from the competition pool and we're going to pump the water from the warm-up pool into the competition pool," he added.
The operation should be finished by 07:00 on Sunday, Nascimento said - four hours before synchronised swimming gets underway with the duet free routine.
Nascimento said the diving pool and then the bigger pool next to it turned green because a contractor added 80 litres of hydrogen peroxide which neutralised the chlorine.
Hydrogen peroxide is used in the cleaning of swimming pools but it should not be mixed with chlorine, Nascimento told reporters.
"Our contractor's failure is our failure," he said, adding that the effect of some 120 athletes jumping into the pool had compounded the problem.
Divers said it had no effect on their performances but water polo players using the larger pool complained of stinging eyes as extra chlorine was pumped in.
"Synchronised swimming requires clear water for refereeing and clear water for athletes to see each other so we're going to change the water," Nascimento said.
"This (replacement operation) will be done overnight. The warm-up water is in perfect condition with ascetics, visibility and transparency for the athletes to perform at their best."
Rio Olympics spokesperson Mario Andrada said the "radical measure" had been taken after discussions with aquatics body FINA.