Federal Environment Minister Steffi Lemke (Greens) relies on cooperation with Poland to clarify the fish kill in the Oder.
After meeting her Polish counterpart Anna Moskwa on Sunday evening in Szczecin, Lemke said that solution-oriented discussions had been held and that "good, common steps" had been agreed.
These include, in particular, improvements in the information chains.
There have been omissions in the past few days.
The German side was initially informed by anglers about the fish kill.
The aim is now to minimize damage, to inform and protect the population and to identify who caused the environmental disaster.
Lemke also thanked the full-time and voluntary helpers who have already recovered vast quantities of dead fish from the border river.
"It is clear that we are facing a really bad environmental disaster," said the minister.
The effects, which could possibly extend for years, are not yet foreseeable.
Unusually high oxygen levels
Meanwhile, according to the Polish government, laboratory tests of dead fish from the Oder have so far not revealed any toxic substances that have caused the fish to die.
The fish were examined for mercury and other heavy metals, said Poland's Environment Minister Moskva.
In the coming hours, the fish samples would be examined for a further 300 harmful substances, including pesticides.
In addition, fish corpses are to be dissected and the behavior of the fish examined shortly before they die.
Moskva said water samples showed elevated oxygen levels, which is unusual for the summer period and low water levels.
It is possible that the water was oxidized.
This could indicate that the fish kills may not have a natural cause, but that there is a perpetrator who released substances into the water.
The death of fish in the Oder has been worrying people who live on the river in Poland and Germany for days.
According to government information, Polish authorities had already received the first indications at the end of July that masses of dead fish were floating in the river.
The cause of the fish kill is still unclear.