it looks almost as if something is eating it. i havent seen anyting like that in my tanksthough so i am not for sure about that. try to watch the pond snails on your plants and see if they are nibbling on the 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.
Jeff, thanks for the recommendations. I prefer liquid because of its simplicity and I have very small tanks. Can you provide a link as to where I can purchase the dry ferts? Or maybe a common store that sells it. Thanks!