Frogspawn and hammer corals losing head

[Coleus]

Well, my frogspawn and hammer corals are dying.

They keep losing one head per day. It is not brown slime but shrink and fall off. I notice this problem after i start doising my vodka. I started out a very small amount but increase the dosing over a week.

I have stopped the dosing now but they are still dying. Someone told me that it is because my potassium is low, is it true? I am not sure what is going on anymore.

All my other corals are doing fine except for brain corals but I think they are being picked by the angel fish.

Any suggestions of what i need to do. My water parameters are

Mg: 1380 - 1400
Alk: 10
Calc: 420-440
Nitrate: 15 (I knows it is high but i has been like that for long time)
ph: 8.2 (test kit) 8.7 (ph probe, calibrate it twice and still the same result, probably broken probe)

Potassium: no idea because no test kit