And now also the solution to the problem: The expert approved the Schwarze Wanne (only a Zebra-Wanne was ordered and we have 10 instead of 5 years warranty).
And now also the solution to the problem: The expert has accepted the black tank (only a zebra tank was ordered and we have 10 instead of 5 years warranty.
What is a zebra tank? If I see it correctly, you built the basement with prefabricated elements. Were the element walls poured with waterproof concrete? A black tank can be absolutely sufficient if the soil conditions allow it. Only so far there has been no statement about the groundwater level or possible other external water impacts.