Ro_land

I have a 200liter freshwater community aquarium with a year and a half old setup. In the last two months I found some black/grey formations mostly on my foreground plants, close to the bottom like Glossostigma and Sagittaria (see photos) but also on the wall of the aquarium. A part of it comes off by scraping with the finger.
The aquarium is not overpopulated (few rasboras, neons)
I use CO2 addition, macro (KNO3 and KH2PO4) and micro nutrients with 5x30W light (2x865, 2xOsram Fluora, 1x830).
My test readings are:
KH = 8
PH = 6.85
PO4 = 1.0 ppm
NO3 = 15-10 ppm
Fe = 0.1-0.2 ppm




Has any of you seen something like this? Any ideea?