One has to take under consideration an aspect of antagonisms between particular elements in plant nutrition. An excess of some nutrients can cause deficiency of another.
Excess .......... Deficiency
N .......... K
K .......... N, Ca, Mg
Na .......... K, Ca, Mg
Ca .......... Mg, B
Mg .......... Ca
Fe .......... Mn
Mn .......... Fe
Now, that you have another angle to resolve your, obviously, nutritional problem ( to this, muddle your picture with pH and water temperature, as well as particurality of this plant), go figure...