What you want to do is very simple. You need to find out what is in your
tapwater - how much nitrate, how much carbonsate, how much magnesium and
for other chemicals.

Then you need to establish what you want your tank to be. Again, how much
nitrate, phosphate and so on.

Now you buy some Potassium nitrate, Potassium sulphate, Magnesium sulphate,
Potassium monophosphate and an iron and trace elements mix.

Make up stock solutions of them in distilled water.

Now, knowing what your bare tapwater is, and what you want,
you can ad these chemicals one at a time from solution and attain
the levels you want.

I follow you till there and I do it for long !
However, surely due a mistake of mine when I reached 2000 mg/l of K
in my tank, I have difficulties to bring it lower than 60 mg/l
surely due to the fact my trace mix shall contain some K too...

It shouldn't. Where were you ading potassium.

If you chnage half he water once a week it'll take care of any
imbalances although you may need ot make two 80% water changes
to get rid of any current exess wastes (like too much K).

As I have lots of plants, I had only Magnesium nitrate and Calcium nitrate
to feed them in nitrates, raising the GH accordingly !

Now I add Nitric acid diluted at 2%...
And I change 25% of the water each week !

I'd go for 50. This limits any concentration of chemicals
you ad per week to 2X their weekly dose as an absolute
upper bound.

GH and KH slowly go down :-)
Unfortunately not K, so I will limit the trace mix

Which Fe level should be OK ?

3ppm unchelated and 3 ppm chelated is what natural cryptocoryne
waters in Asia are (Host and Kipper). I use that and it seems to be fine.

Cheers,

--
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