Is this a malnourished plant?
Disher, I don't know why you prefer liquid, but I just started dry ferts and LOVE it. K2SO4 is what you want for just potassium dosing, but dry nitrates add potassium as well. (KNO3)
Dry ferts are way cheaper than liquids (I was a Seachem lover previously) and sprinkling in a half of a 1/4 tsp of something is drop-dead simple.