Look at the water company chart where it says Physical Properties.
Go down to Hardness (mg/l) CaCO3 and it is 18.
This is very low GH.
Get GH booster. This supplies calcium and magnesium, and many GH boosters may have potassium.
You can use baking soda for carbonates if you need to raise the KH. Carbonates are the buffer that will stabilize pH.
I have had similar experience with KNO3 and KH2PO4: To properly dose K using these I have to add too much, so the NO3 goes sky high. Get the K2SO4 so you can dose K without having to overdose the N or P.
My tanks were low tech, Excel for carbon, low light, and heavily stocked. When I dosed NO nitrogen fertilizer, I had to do water changes to keep the NO3 low. Since fish food also contains P I assumed the same was going on with P: The tanks had plenty without adding fertilizer.
Then I improved the lighting and had to dose a little bit of KNO3, so I also dosed a little bit of KH2PO4.
In all these, there were holes in the leaves and discolored leaves that indicated deficiencies of K and Fe, and these problems went away when I dosed K and Fe, so I kept on dosing fairly high levels of K2SO4 and iron chelate even when I increased the K from KNO3 and KH2PO4. I was not adding that much of either of these to fully supply the K needs of the plants.