best test kit for potassium K

[abc]

I can't seem to get rid of my blue-green algae, no matter what I do. I've tried:

1) maracyn 1 treatment but the algae comes back
2) less light, more light, no light, different and various ways to find the right lighting
3) upped the nitrates and phosphates method

To make a long story short, I want to break away from the EI method of fertilizing and try to find my ideal ppm for nitrates, phosphates and potassium and see if I try to fine tune the water parameters, I might fix the bga.

That said, how do you measure potassium? when I searched the net, I found some kits but they are expensive for this one element. does anyone have a cheaper way than this kit?:

Amazon.com: Reef Care Potassium Pro Professional Test Kit - 40 Tests: Pet Supplies

Amazon.com: Reef Care Potassium Pro Professional Test Kit - 40 Tests: Pet Supplies

[abc]

if no one knows of a good K test, how about a different question:

if you are not going to do EI, what is the "ideal" waterparameters for a fully planted tank with HC carpet and mixed high light and medium light plants?

I'm looking for a long term solution to BGA. I could drug the tank with maracyn and everything goes away, but it always comes back. I'm struggling to figure out how to find the right balance of fertilizers

[Kudaria]

The only potassium kits I'm aware of are the one you mentioned and the one made by Elos. Both retail for around 40$. It simply doesn't seem to be a very popular test.

On the plus side you would probably only need to test once a week for a few weeks to determine the dosing you needed and then probably only test once every two weeks or a month.

I believe greenleafaquariums has some links to some excel charts to help you with figuring out how to make your fertilizers.

I too am not interested in doing 50% water changes every week. That's just too much water for me to lug around the house and the tank is not at all convenient to the kitchen. Thus I too am looking at trying to figure out how to just put in the amount of fertilizers that my plants need so I can do 25% or so water changes instead of 50%.

[ThatGuyWithTheFish]

3 day blackout and more nitrates.