Use time for tea too short

In the first 7 to 10 days, we were pleased with cooked black: no film and no edges.
Unfortunately, the filter effect was strong after a week and the third week we were able to find no difference for unfiltered tap water. The calcium deposits in the kettle are, however, even after 3-4 weeks still significantly lower than with unfiltered water, but as I said, the black had another film and unsightly deposits in pot and cup.
We have up to this assessment uses the 4 cartridges in BWT 3 + 1 pack. The results were comparable. In between, we have a Britta Maxtra filter used. This filter we could use about 2 -3 weeks for clear black. We will test more filters and hope in the Berlin area again one day the tea quality of the northwest German area to reach.