Originally Posted by Wasserpest
You can't really test for Potassium. Just take Chucks calculator, plug in 1 ml water, your tank size, and then play around with the teaspoons and see how many you need to get to 20 ppm.
After a gradual initial addition to bring it up to that level you will be dosing just for the water that you change.
I'm really confused now. On Chuck's site it says "this is the total level you should target for the tank. For nutrients like Potassium and magnesium, I add enough so that each week, I'm adding close to this amount." I was assuming that you start from scratch each week - so that in my 50 gal tank, for instance, I would add around a teaspoon of K2SO4 every week, which would add around 15 ppm of potassium. Which is correct? Do you add enough for the whole tank each week, or just enough for the water you're replacing? If you just add enough for the water you're replacing, that seems to suggest that your plants aren't using up what you put in the tank the week before.