Potassium fert.

[Otterfun]

Hi,
My boss gave me a bottle of potassium when I mentioned my leaves were yellowing. I put like 5cc in the 5g tank and in one day of 8 hrs 6500K LED, the pygymy chain sword runner sprouted by 1/2".

My tank does not have a betta (Fred died in QT tank). It has 4 Amano and 1 Golden Apple Snail and I intend to continue cycling with them in it.

I have been doing water test everyday and changing water when nitrites exceed 0.25. My nitrates are 30-40. I tink my ammonia has been below 0.25 for at least a week. I just added 5cc of Stress Zyme in the tank.

So my question is:
Do I need to monitor the "saltiness" or level of Potassium in the water?
If so, is there a test kit for it and what is the recommended brand for liquid test, if any? API??