A sudden change in alkaline levels in the Olympic diving pool in Rio has been blamed for turning the water a bright shade of green.
The problem also affected the synchronised swimming pool to a lesser degree on Tuesday with athletes puzzled and concerned.
But Rio communications director Mario Andrada said the health of competitors was not at risk with checks confirming chlorine and PH levels were within required standards.
"Mid-afternoon yesterday there was a sudden change in the alkalinity of the diving pool," Andrada said.
"We have treated both pools during the night and the alkalinity levels have already improved.
"We expect the colour to be back to blue very shortly."
Andrada said a test event was conducted in the pools before the Olympics but officials should have done more intensive testing.
"We probably failed to note that with more athletes the water could be affected in different ways," he said.
A team of independent experts had also checked the pools and ruled them safe, Andrada said.
The international swimming body FINA said water tanks at the pool ran out of some chemicals used in the treatment process.
"As a result the Ph level of the water was outside the usual range, causing the discolouration," FINA said in a statement.
It said its Sport Medicine Committee had conducted tests on the water quality, and found there was no risk to the health and safety of the athletes, and no reason for the competition to be affected.