Thanks for the PM directing me to this thread. Its a good discussion with lots of useful input from people who know what they are talking about. I'll share a couple of thoughts based purely on my own experience.
Unless you were calculating PO4 based on P instead of PO4 (as you mentioned in pm) I do not believe 4-6 ppm/week is enough to cause problems. Back when higher PO4 caused issues in mine I was still using csmb and dosing extremely low levels - .00187 3x per week + the same ppm of DTPA and later gluconate at the same along with it. Yes those zeros are correct. Some plants grew wonderfully at that level but many did not. I was never able to strike a good balance using csmb, but that is a different subject
Since rolling my own micros the sweet spot for most of the 120+ species I keep seems to be in the .1-.2 ppm Fe 3x week range. This in both sand tanks with KH 5-6 and aquasoil tanks with KH 0-1 (all the time because I lower it with HCL)
The soil tanks can get by with less micros and they can also tolerate more without causing issues. I dont know why, but there is a much narrower margin for error in the sand tanks. its not entirely related to KH either because I ran 1 and 2 in the sand tanks for proably a year as well. Didnt change the result as far as that goes.
Back to PO4, since ditching the csmb Ive been able to use higher (and more efficient) micros, and limiting PO4 is not shown to be a good thing at all, quite the opposite actually. 4-6 ppm/week tends to be the sweet spot.
When you mentioned the plants that are currently having issues - "Hygro Corymbosa, AR mini and regular and Limnophila Belem, and some Acmella Repens stems. L Rugosa"
Those exact plants are the first to go south for me when K is too low. Me, Vin Kutty and a couple others went through some lower K experiments last year and all got the same results. It didnt help the species we were trying it for (sensitive Lythraceae) and we all saw issues with the plants you listed in particular. It was uncanny to read that list just now because those exact ones are the first to go south when K is too low. The main thing we all learned from the low K experiments is that those plants specifically are great indicators for having enough K. I can add Buces to the list as well.
And interestingly enough, the problems with low K rarely show up as the familiar pinhole symptoms we were all taught to look for. Except for Hygro sp which are already going south at the tops before it happens. For years I always assumed no pinholes meant there was plenty of K in the system. Not true. The first sign the K is too low is usually pale and stunted new growth
But I will say this, larding on the K isnt good either. Did that for years too because, well we all know K isnt "toxic." But too much starts to interfere with the plants ability to utilize other nutrients. All nutrients compete with a few specific other ones for uptake by the plants (thats a broad generalization as far as exactly what happens in every case)
To put in ppm terms, 35 K per week was causing various issues that I always thought were due to something else. Going down to 20-ish flipped a switch where several things got immediately better. Including species that I didnt even realize were sub par in the first place. But then say 15/week or less, that's not nearly enough.
Im not a big believer in specific ratios beyond more than a general guideline. But there is a definite balance to strike between everything. Its a main reason why @Jeffww
has the best results with 1 or 2 ppm Fe per week, and somebody else might have better results with 1/10 of that. They have both struck a balance. When the low dosing guy tries Jeff's level and things go south, its not because X ppm is toxic or whatever. Its because raising it that much threw things out of whack. I guess you could say 'toxic' is as good a word for it as anything but that's not actually the problem.
This is also why Jeff sees issues when he tries to dose less. Balanced is balanced whether its high or low. And because everyone's parameters are so different, what equates to balanced in one tank doesnt always translate to another one. And of course there's always the underlying factor that certain plants simply need more of a particular nutrient than others, or may have a low tolerance for high levels of something. When it comes to fish and livestock we gladly accept that different species need different parameters. The plant crowd has a harder time accepting that different plants can have different requirements too.
So there's a War and Peace novel of anecdotal experience and idle musing. Maybe somebody will find a thing or two useful in it.
Keep us posted how it goes!