[Edward]

Few days does nothing, plants take time to respond. The PO4 of 4 ppm is unnecessarily high, I would keep it lower. You don't mention Ca and Mg. TE trace elements are needed in very small quantities only. The best way is to dose TE when white new growth appears. Extra TE may be toxic to fish and plants. Fe is also a trace element.
Nutrient consumption rate is less important then plant health. Concentrate more on better looking plants. Healthier plants resist algae better and don't require frequent trimming.

Edward

 

[Edward]

No need to add Ca, Mg - tap water is ~17 PPM GH so I have to mix with RO.

I think you mean 17 dGH. Even though this is hard water, it doesn't have to have enough Mg.

I dose 2.5 ppm Fe per day - I did a short survey and that's what folks are dosing these days.

Toxic overkill.

And my question was actually - if I do provide with all the necessary nutrients - is it that it still takes time for the plant to first deplete it's own reserves before consuming nutrients from the water column?

No. I don't think plant would be depleting it's nutrients if there was nutrients in the water column available.

Is it true that when CO2 was low the plant couldn't use as much nutrients for photosynthesis and therefore stored it and therefore now doesn't need new nutrients from the water column? Hence low consumption rate?

Yes. Plants can store huge quantity of nutrients that can last for few months. Problem is, the stored nutrients are not at the proper ratio.
Secondly, plant using it's own supply decreases it's ability to resist algae infestation.

Edward

 

[Edward]

Sorry, let me rephrase it.

The best way is to dose TE when pale new growth appear and continue this dosage to maintain healthy results.
or
The best way is by observing plants for signs of deficiency, pale new growth. If that happens increase the dose.

Perhaps there is a difference between TE products. Why my plants don't show TE deficiencies at 0.002 ppm a day? Some people dose TE once a month and don't get any deficiencies neither.

Edward