From the numerous symptoms that are observable from the picture, the water is most likely hard, likely very high Ca and very low B.
L. "red" - sides of leaves curl heavily under which is a symptom of -B. -B can be induced with very high Ca concentrations.
Crypts - leaf margins are wavy which is a symptom of -Zn. Same with -B, it can be induced by high Ca concentrations.
The algae colonizing the leaf margins are likely a symptom of -Zn.
None of these symptoms are a result of -K.
I'm pleased things have worked out. That looks really good. Your Apistogramma looks happy and your moss is great.but the tank is free of algae and all the plants are growing well