> I wonder what such stories, if true, teach us for the future.

That if you're looking to maintain [water] quality, you need to have monitoring in place.

Switching to the new provider was a good idea, not a bad idea. There is no way to keep track of everything that might have happened in the past that might be affected by unspecified changes in the future, so there can't be an expectation of doing it either. But you can watch what kind of results you're getting.