KH2PH4 is safer fish I target about 1ppm (1mg/L) K2SO4. For macros I have also used Ca(SO4), CaCl2, MgCL2, MgSO4, and NaCl, KNO3, K2SO, KH2PM4 and CO(NH2)2 (urea) At different levels and not seen any harm. The macro nutrients for plants are Nitrogen (N), Potassium (K)Calcium (Ca), Magnesium...