There's a prevailing opinion that high level of nitrates (tens of mg/L) are harmless. While I agree that they hardly inflict noticeable damage on animals, I think differently when it comes to aquatic plants.
I've got a theory on plant nutrition, and hereby I present it to a discussion.
In nature, phosphorus and nitrogen are usually the limiting nutrients. My observation as well as literary sources suggest that almost all plants tolerate low to very low levels of P and N quite well (there are exceptions, par example Echinodorus, some free-floating fast growers, etc.); they adapt their growth rate but suffer no deficiency symptoms. Yet care should be taken not to let N and P fall to zero. Complete lack of phosphorus, in particular, is an invisible killer.
Deficiency symptoms appear not when nitrogen is limiting, but only when nitrogen is in excess relative to other nutrients. Excess of phosphorus seems not to have these consequences. Two points lead me to this assumption:
(1) Some aquarists clearly overdose phosphorus without any apparent drawbacks.
(2) Nitrogen forms an absolute majority (in molar terms) among all nutrients.
In short, nitrogen is the nutrient regulating the growth rate. Light and CO2 certainly exert influence on this but they create just organic carbon which can be used as an energy source, energy reserve, or for growth - if nitrogen is present.
Most aquarists willingly or unwillingly (when their source water contains nitrates) face a situation when nitrogen concentration far exceeds that of natural waters. (I've experimented with "natural" levels of nitrogen - lower than equivalent 1 mg/L nitrate - and faced practical difficulties. It was near impossible to keep nitrogen that low and not to be repeatedly falling to zero at the same time. In this, I've relaxed my strictly "natural" approach, and do not recommend it.)
With nitrogen aplenty, plants want to grow but face difficulties when any other nutrient is in relative deficiency. That's the source of deficiency symptoms. All of them? I rather suspect so.
While reading discussions in this forum (and not only here, of course) I often wonder why so many aquarists struggle with deficiency symptoms while dosing much more and much more often than I do. I admit strong lighting and CO2 injection play their roles, no doubt. I wonder whether CO2 injection could induce deficiency defects if nitrogen is limiting nutrient? I'd really like to know.
Iron, for instance. I'm using only a weak chelate (citrate), and often not even that (iron (III) chloride). My regular amount is 0.017 mg/L (iron content) and I dose it (much) less than once a week. True, some of my plants suffer from iron deficiency, but exclusively in alkaline water (pH = 8), which I accept as part of my experiments. In such cases, adding more iron does not help, anyway. (Advanced chelates might work, though, I've never tried.) In acidic water - all high-tech tanks are in an acidic range, right? - I seldom notice iron deficiency, and can fix it invariably and instantly with one dose of iron citrate or iron chloride.
So, my suggestion is that moderately low nitrogen (2 to 6 mg/L eq. NO3) is the way to avoid most nutrient deficiencies. I'd like to discuss this theory with you, esp. whether you've made any observations which support or contradict it.
(Another side effect - a positive one as I see it - is a reduced growth of fast growing plants, and reduced need of trimming them as a result.)