You cannot use KNO3 purely as your source of K without using something like
KCl or K2SO4

Well many folks have for quite some time.
Also see APD recent post where I discussed this.

It is not correct to say that there is 4x K compared to NO3. (I presume you
are using molecular mass)

Well actually it is. But I did not say that with respect to NO3. I
said 4x as much with respect to N relative to K+.
Re read the post.

It is quite correct as far uptake and plant biomass. A plant has
roughly 1.5% dry weight as N. It has roughly 1% as K+. See Epstein
1972 for a reference to support this dry weight mass.

N = NO3 when using KNO3.
So taking the N moles it becomes 14g mol N/62g mol NO3= .22 moles of N
in each mole of NO3.

So now since it's 1.5% vs 1 %, this becomes 1.5 N/1 K= 1.5/.22 moles=
6.8x more need per mole.

Since we use KNO3:
KNO3 is 101.1g mol. K+ represents 39% of this mass. N represents 61%.
61/39= 1.56. 6.8/1.56= 4.3 x more K+ than N needs per mole of KNO3
added.

So yes, it is correct in saying that you don't need to add more K+
sources, even if you consider fish waste and other sources of N.

Unless you are adding susbtantial inputs of NO3 from fish or from the
tap water, most folks don't need to add K2SO4 or KCL.
Which is what I said to begin with.

Even if a plant has more, needs more K relative to N and other
variations are considered, 4.3x as much is such a large factor that K+
will not become limiting in most planted tanks using KNO3.

If you dose KNO3 to obtain nitrates of, say, 10ppm, your K will be less than
10ppm. Now, I am sure this is fine for plants to grow, but your opening
statement says "Keep the NO3/K+/PO4 in good shape, 10ppm/10ppm/.5-1.0ppm"

To achieve your quoted ratio, you will need KCl or K2SO4

After a couple of doses it will be close to this since the amount of N
actually used relative to K is higher, leaving more K+ behind as the
plants remove more N. Folks will often re set their tank(some less
frequently than other etc which would mean more build up of K,
relative to N)

It's an ideal ratio that adds K+ to a slight excess but few test for
K+. If someone is off by a few ppm on the K+ either way they are fine.
A Generalized ratio is 10-1-10. There's some excess as far as K+ using
this ratio.

None of these levels/paramaters are set in stone and are only a snap
shot in time, if a range is kept, then the plant's needs are met and
they grow well, they do not have to have a precise ratio or level.
No need for folks to be obsessive or freak out if the range is off by
1-2ppm with K or N.
It will not make or break plant tanks using KNO3.

My point is that most folks don't need to add KCL and K2SO4 if they
use KNO3 unless they have high NO3 tap water or a high fish load, non
CO2 methods generally don't use KNO3 since less mainteance and less/no
dosing besides fish food is often the goal there.

But will adding some extra K+ from K2SO4 or KCL hurt? No. My point is
that's most likely not needed though and is one less thing to buy and
dose.

Regards,
Tom Barr