I got this Aponogeton Madagascariensis sp. as a gift from a customer. The plant is in a small sucker cup temporarily in my nursery tank as I don't have space at ground level in both of my tanks. It grows rather fast to my surprise and produces a leaf every week. Plant has been doing fine for the past 2 months with nice spotless green leaves but I noticed some black/brown spots developing since last week. They don't look like algae but could be, not sure. The dark spots are not soft to the touch as initially I though it could be decaying matter. I am unsure what is happening as all other plants look fine. In case it is algae, what is less aggressive to this lace plant; Hydrogen peroxide or Seachem Excel? If it's not an algae any pointers what is happening to the plant?