If you ever feel something is deficient, add more, a very simple solution to rule out such possibilities. Wait 1-3 weeks etc. If nothing improves, then you know it's something else.
This is surely the easiest test.
If adding a potential cure does not cure the problem, then the problem is something else.
It sure worked for my tanks.
The problem looked like potassium deficiency, so I dosed K. The problem went away. It comes back when I slack off dosing K.
The plants are the best test.