Black Stuff

[Frederick]

Hi all.... I dose with CO2,Tropica non nitrate fertilazer,dose with easy aqua,lights on for 6 hours per day all water parameters ok....however... many plants and the rocks are covered with a black covering...not slimy and not long, just a covering.The plants effected mainly are stem plants,anubias etc.
Could anyone in the forum please tell me what it is and how to get rid of it.
Her"s hoping......Frederick

 

[DrRob]

Photos would help, as always.

Initial answer would probably lean towards flow being too low, or CO2 fluctuating to much.