The Polish fire brigade has already removed 100 tonnes of dead fish from the river Oder since Friday.
“We’ve never had an operation of this magnitude on a river before,” said Monika Nowakowska-Drynda of the Polish fire service’s news agency.
About 500 firefighters are involved in the clean-up operation.
The cause of the fish kill in the river, which flows through Poland before forming the border between Poland and Germany, is still unclear, but authorities fear that chemicals are involved.
Last week Prime Minister Mateusz Morawiecki said enormous amounts of chemical waste may have been dumped into the river. Water samples have been examined in Poland and Germany, and Poland has also sent samples to laboratories in the Czech Republic, the Netherlands and Great Britain.
The European Commission has offered its help in the investigation into the cause of the disaster.
Residents and fishermen on the Polish side of the river made the first reports of massive fish kills in late July. The German authorities charged that the Polish government had not informed them, so they were surprised when the dead fish suddenly started to appear on the German side.
Poland's government has also been taken to task by critics who say it did not take measures fast enough.