At this point, it could be deficiencies or toxicities.
Most often deficiencies would be of the macros-
Please post NO3, GH and any other test you have (Phosphates would be good) at the beginning of the week (a few hours after a water change- let the water get thoroughly blended, but before dosing anything) and at the end of the week, right before a water change.
Toxicities are usually micros. These are minerals that plants need very little of, and fish food supplies quite a lot of. Except for iron. Please post any tests you have that might help (probably just iron- I do not even know if there are hobby level tests for the others. Maybe copper?) and TDS. (Also at the beginning and the end of the week)
Here is how I would start trying to figure this out.
What ferts are you already adding from other sources?
Tap water with high GH supplies Ca and Mg and perhaps other minerals. Low GH (under 3 German degrees of hardness, especially if it drops through the week) hints that perhaps you are seeing Ca or Mg deficiency.
Are you adding minerals from other sources? Buffers, pH adjusters, dechlor with 'electrolytes' or anything else? Read the label to see what you are adding. Test TDS before and after adding some of whatever you are adding.
If you were not dosing at all, do you know what your NO3 would be? This would tell you how much fertilizer you are getting from fish food. Fish food is a reasonable source of N, P and most traces. It is low in K, Ca, Mg, Fe. If there is a moderate or higher stocking level in the tank, meaning you are feeding the livestock quite a bit, then you are adding ferts in this way. Not as much as a high tech tank needs, but some.
If NO3 is ...
Very low or 0, then the plants are taking all the fertilizer the fish food has, and probably want more. Full EI is good.
Moderate, then the plants are using most, but not all the ferts from fish food. I would first supplement with K, Fe and make sure the GH is good (This is a test for Ca and Mg). Might not need full EI, and it might lead to toxicities of traces if the plants do not need this high level. Supplementing just the minerals missing in fish food could make the plants grow even faster, and then they would demand more of all the nutrients.
High Then the plants are not even using all the fish food fertilizer, so I would just supplement K, Fe and make sure the GH is OK. Do not need full EI.
