Originally Posted by broujos
These are definitely black brush algae.
They may, in the long run, kill all plants, but can be eliminated by a combined attack.
If plants and fish in your tank come from acidic waters, with low hardness, you can act as follows.
Fill the tank with a mixture of tap water and RO water (you will need some count).
Lower and stabilize the pH to 6.5, by filtering with black peat and administering CO2 (carbon dioxide) with a diffuser.
The amber color of water, produced by peat, intercepts most wavelengths necessary for the growth of algae, but completely useless for the plants.
Use 18.000° K fluorescent lamps.
Use a powerful external filter, powered by a centrifugal pump and filled with a sintered glass filter medium.
This material allows the growth of anaerobic bacteria as well, which can destroy the nitrate made by ordinary aerobic bacteria.
Of course, the usual rules of good management of an aquarium (a few fish, many plants and regular water changes) must always be observed.