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[APD] KNO3, nitrates and test kits

Hi again,

I have something weird going on in my tank. I'm just starting to use PMDD
for fertilization in my 55 gal tank and there is something with KNO3 and
nitrates which I can't figure out yet.

I used to have about 20-30 ppm nitrates in the water from bioload before I
got into PMDD. Since the nitrates are elevated, I made the mix without KNO3
for now, just trace mix, MgSO4 and K2SO4. I started dozing it for a few days
and nitrates went to almost 0.

Then I created just KNO3 solution with recommended concentration (I used 1/2
tbsp per 250 ml water). I doze 10 drops of PMDD daily, so I decided to use
half the recommended amount of KNO3 for now and added 5 drops of the KNO3
solution along with 10 drops of PMDD yesterday in the morning (after
checking the nitrate level being around 0). After I came from work the
nitrate level was about 60 ppm (according to my test kit)! I was quite
surprised but didn't do anything. Today in the morning I tested the nitrates
again and they were almost zero again. I added another 5 drops of KNO3
solution and you guessed it, in the evening I also have over 60 ppm (the
test reaction is deep red). It's seems even more than yesterday.

I thought that maybe I did something wrong and created the solution too
strong. Or maybe the chemicals were mixed up. So I get just tap water and
added a drop of PMDD to it and tested it for nitrates. There were no
detectable amount. Then I added a drop of KNO3 solution there. Surprisingly,
the test kit just showed a trace of nitrate (less than 5 ppm). Then I added
5 more drops just to be sure. And it showed just about 10 ppm or so.

Now I'm missing something. Does KNO3 release NO3- slowly? As K is consumed
by plants, will NO3 ions be left and detected by the test kit? Could there
be any other explanations for what I'm seeing? Can light play some role
here? Can just a few plans consume 60 ppm of nitrates overnight (without

I will test the water in the morning and if nitrates are low again, I won't
add KNO3 and will see what happens in the evening tomorrow.


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