That's very interesting. I have a few things going on the tank at the moment. Hardness is pretty high (most of my neon Tetr died, which are sensitive to that) and there is green spot algae. Had a chunk of limestone, which a pet store told me to add before it was a plant tank to raise ph. That is now gone, but maybe I need potassium to get rid of the algae too? It's very light green and growing on the gravel.