My Amazon Swordplant has this slow growing black algae on it that is driving me crazy. It takes about 2 weeks to infest a leaf bad enough to pull it. This tank has never had any real algae problem but this algae has decided to spread to my valisneria and bannana plant. I have some green spot algae but other than that, the situation is rather balanced. 2.5 wpg with DIY CO2. I does with K and Seachem Flourish weekly. Nitrates are usually around 20 ppm before a waterchange and 15 ppm after. This algae is flat against the plant and comes off if I scrape the leaf with my fingernail. I assume that it is some form of red algae. Any idea how to rid myself of it?