06-21-2011, 12:47 PM
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This is a very stubborn type of algae, Black Brush [actually, it is a red algae but so dark it appears black]. I have it in some of my aquaria. I agree, on wood or rock it is very natural. Once it starts getting on plant leaves, it can kill the leaf and eventually the plant.
It frequently occurs on slow-growing plants such as Java Fern. Anubias is another favourite host. In both cases, this is probably due to the slow growth and toughness of the leaves. Something that might help is to have either of these plants in semi-shade, not directly under the tank light. They do better with shade anyway, so floating plants are easy to solve this problem. I find this algae often increases first closer to the light.
The danger is that it will spread to other plants. I have found reducing the light helps to keep this under control. I have previously seen it increase in summer, undoubtedly due to longer and brighter days, so I now keep my fishroom windows heavily covered during summer.