Nitrates @ zero = bad. If true, this will lead to terrible plant growth and algae.
There are 3 macros (some count a few more)
All 3 are required in "relatively" high amounts, although more <> better. Looks like you should buy a pound of KNO3 and one of K2SO4 (for potassium) since you have enough PO4.
Then start adding KNO3 in small amounts and see how the tank reacts after a couple of weeks. Your PO4 is fine, and don't worry too much about GH and such.
Actually that was for the tap water itself. The tank has 5-10ppm nitrate currently.
Well I have kno3. I bought a pound of Kh2Po4 for potassium but I am guessing since its potassium phosphate = bad and the K2So4 is better for my circumstance.