Experts in the Czech Republic, the UK and the Netherlands are helping the investigation into the hundreds of tonnes of fish found dead in the Oder River.
The death of thousands of fish in a river bordering Germany and Poland has sparked a political row between the two countries.
Scientists are still trying to find an explanation for the mass die-off, which has seen over 200 tonnes of dead fish recovered from the Oder River.
Poland's environment minister has accused Germany of spreading "fake news" about the environmental disaster after it reported on high pesticide levels in the waterway.
"In Poland, the substance [pesticide] is tested and detected below the threshold with no effect on fish or other species and no link to die-offs," Anna Moskwa wrote on Twitter.
Experts in the Czech Republic, Britain and the Netherlands are helping with the investigation as water samples have been shipped off to labs for analysis.
Some researchers say that toxic algae may have killed off the fish, but other theories including a possible chemical spill are also being explored.
Poland's government is offering one million złoty, or €210,000, to anyone with information on who, or what, caused the disaster in Poland's second largest river.
The mass die-off was first detected when locals came across thousands of dead fish in waters near the village of Widuchowa in western Poland on 11 August.
Some residents said that workers deployed by the government to remove the animals were not prepared for what awaited them in the river.
The stench around the waters was so bad that some vomited during their work, locals told Reuters.