Environmental officials are investigating the fish kill. Biologists say it was likely caused by raw sewage making its way into the water, as has happened before.
Rio has many problems with water pollution. Many of the waterways near where Olympic events are to be held have been found to be polluted.
Rio's Cedae sewage company says in a Thursday statement that all of its treatment plants are operating normally in the region where the dead fish were found.
But it says it's sending out teams to see if any pipes are leaking.
Edited by Steve Wilson