What causes black beard algae

[Nik_n]

I have a 180l tank, which is relatively lightly stacked at the moment. It's heavily planted with some rocks and driftwood and I have been struggling with BBA for a while now. I tried soaking slow growing plants, such as anubius in hydrogen peroxide, which did kill all the algae but it did harm the plants a bit too. However it came back soon after and covered rocks and driftwood. I don't overfeed the tank, I test the water regularly so the only problem I could identify is lack of flow. The internal filter is an Eheim 240 (don't know the exact name of the top of my head) and the flow from it is minimal. I was planning to add a power head but since I have slow swimming fish such as appistogramma and angelfish I don't think that's the right option. 

The main thing that bothers me is that I'm not sure that low flaw is the only factor since I know people successfully run tanks with little to no filtration or water movement and do not encounter this problem. I also keep the light on for 6h a day but the tank is loacted in the center of the room with windows on both side. Could this be the problem or is there another influential factor 

 

[Mmiller2001]

Some Causes are

High flow areas

Fluctuating CO2 

An abundance of organics in the tank 

Overall poor plant health

Dead spots in circulation

I would increase the turnover in the tank but try to create subtle or indirect flow. Spray bars work well to reduce flow but keep high turnover. Increase water changes to keep organics down and make sure mulm isn't building up. Make sure plants are in good health. Remove yellowing and dying leaves when needed.