Well, toxicity testing in general is interesting to me.
Given I was/am one of the few stupid enough to test such stuff without fear.........I am able to learn more than the fear and myth crowd.
This applied to algae.
This applied to plants
This applied to fish
This applied more recently to Cherry shrimp, and then CRS mid/low grades, then high grades.
This applied to less light, how low can you go?
I am honest in my abject ignorance, and then going about trying to answer some of these questions. Many seem to think they know a lot....but really just repeat.
Seek questions, not answers.
We really do not know a whole lot about color, we know a few things based on some observations, but they seem conflicting to me. Still, we can hypothesize and try and rule things out, and narrow our choices down.
I do not think there is strong evidence that low NO3 alone, has much impact on nice red color.
I do not think L aromatica is a good test case though. A reineckii, L pantanal, R macrandra, a few of the Rotala's, Ammannia etc.
These seem better.
I think you'd really have to dose a lot of MgSO4 to get to the toxicity ranges.
We just do not commonly see it. Someone sees a little correlation and thinks there is something to it maybe.........perhaps......and says it's just speculation and 3-4 people later it's become fact
K+ upper ranges and some of these other nutrients are going to behave more like Salt stress models rather than "toxicity" like with something like metals like Copper or Zinc, K+ and PO4 will fall into this group.