After the fish die-off in the Oder in the summer, representatives of authorities and 14 institutes presented their first conclusions on Thursday in Warsaw.
The approximately 300-page report on this should be available on Friday.
Accordingly, 249 tons of dead fish had been taken from the stream that forms the German-Polish border on the lower reaches.
Poland's government had been criticized for its delayed response to the fish kill.
State Secretary Małgorzata Golińska from the Warsaw Ministry of Climate and Environment rejected the claim that "the state had failed".
Also, no companies were named as possible polluters on Thursday.
The prosecutor is investigating.
Katja Gelinsky
Business correspondent in Berlin
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Marek Kajs, deputy head of Poland's environmental agency GDOŚ, said they are still analyzing data from "hundreds" of companies.
"The thesis that an individual company caused this situation with wastewater is wrong." Reports had mentioned the Polish mining company KGHM as one of the causes.
Scientist Agnieszka Kolada pointed out that low water levels and record temperatures have worsened the condition of the river.
Then a high concentration of salt and oxygen was added.
Reported mercury and nickel were present at harmless levels.
Using satellite images, she demonstrated that an algal bloom was occurring.
The fish kill goes back to the alga Prymnesium parvum, which is known for its toxicity.
The algae were found in 78 percent of the fish examined.
As a remedy, Kolada recommended constantly measuring the water quality and publishing the data.
At low water levels, discharge permits would have to be suspended.
The Federal Environment Ministry will present its report separately on Friday.
A joint report announced by Warsaw in August never materialized.
The Greenpeace organization referred to its own samples that it had taken along 550 kilometers of river.
She found the salt values that were most excessive at a KGHM retention basin in Polkwitz, Poland.