I don't know where you heard that you can get sufficient macros from tapwater, but it ain't true. Unless your tapwater is so polluted you really shouldn't be drinking it, which looking at your water report, isn't the case.
You're probably getting enough nitrate and phosphate from fish food, rather than the tapwater. But not enough potassium - which you've already correctly diagnosed. And a good micro fert including iron would help too.
Off the shelf at your LFS, Seachem Flourish and Potassium at doses recommended on the bottle will fix you up quick.