artenwasser for toilet flushing - This should only be used with deep water and appropriate preliminary examination, for example for iron content, otherwise you will ruin your entire installation.
Well-invested 50-80€. It would be really bad if the toilet suddenly turns brown and you have to clean behind it constantly.
But it must be done before installation work, as using well water requires a second water circuit, which also has to be paid for.
What do you understand by deep water? Our well starts with its water column at about 6m, the water is almost free of pollutants and minerals (at least as far as iron and the like are concerned, which would ruin the toilets).