I have what looks like a very dark mold on the older bottom portions of my live aquatic plants. What is this, and how do I get rid of it? I have a 20-gallon aquarium with 65 watts of light. This condition showed up after the aquarium got a brownish algae or fungus that I got rid of with water changes, but the dark mold won’t go away.
Welcome to the wonderful world of black beard algae. Technically this is a kind of red algae, but that classification does not define its appearance. This unwanted algae shows up on older parts of plants and on slower-growing plants. The easiest way to deal with these blackish algae is to do three things. First, clean away what you can, by wiping or pruning. Second, ensure that nutrient levels in the water are low: about 1 ppm for phosphates and about 10 ppm for nitrates. (A word about testing for phosphates. Plants use phosphates that are not part of organic compounds. If your test kit measures total phosphate — organic and nonorganic, which are also called ortho-phosphates — then your phosphate reading can be misleadingly high. You want to maintain about 1 ppm of ortho-phosphate.)
Third, add carbon dioxide to maintain a level of about 25 ppm throughout the period when the lights are on. The level when the lights are off does not matter much. If you cannot add CO2, try limiting the lighting level to no more than 2 watts per gallon.
You also might consider reducing the amount of light you are using, because 65 watts on a 20-gallon planted aquarium is a lot of light, especially if you are not adding CO2 to maintain about 25 ppm. Running an aquarium with that much light makes it much more difficult to avoid algae issues — you can have too much light. You might even be able to just swap out the 65-watt lamp and replace it with a 36-watt version; it depends on the particular ballast being used. A 36-watt tube would be slightly overdriven by the ballast, but nowhere near 65 watts. Alternatively, you could use a different hood or use something to partially reduce some of the light from the 65-watt lamp — a strip of aluminum foil on the cover glass would do the trick.