I have a 20 gallon tank with 2 guppies, 1 small pleco and 3 nerite snails. It also grows about 5 roots of Anubias, some grass and another small creeper. http://s1316.photobucket.com/user/vaioana/media/IMG-6012_zps8n3sken1.jpg.html The tank is fully cycled, with amonia and nitrite readings at zero. I change about 30% of the water every two weeks. (Not sure how to embed the image, but there's a libk to it on Photobucjket) The issue is that I keep getting these algae that cover the substrate and every surface in the tank. They look like dark green / black slime that covers the leaves of the plants and the gravel. Once deployed, it is difficut to rub it off, as it adheres to the surface quite well, especially on plant leaves. It also eats at them around the edges. The grass is completely covered. This is what my tank looks like 1 week after general cleaning. The nitrates are already elevated and the water quality is not great, despite the established cycle. The filter is so engrossed in slime that it has a hard time moving its biowheel. (fresh media with water change). After each water change, I get a nasty bacterial bloom, turning the water cloudy for 24 h. It clarifies well, and the water is crystal clear for a few days, then the algae start to grow and the slime is covering the filter. What you see on the glass is new -- I usually do not have that covering of green yuck, but the nerite snails are doing their best to clean it, Rumba -robot style (some areas clean, some not). The tank is near a window, and it is a MarineWorld kit with domed hood and integrated light. The light is only in the middle and does not penetrate to the bottom when turned on. I added an LED tube to the from of the tank, to help plants thrive. The algae problem predates the LED tube, which I keep off these days to prevent more algae from growing. (It was on for the picture). I also add some Flourish plant food every few days. Final piece of info. The plant was established about two months ago, reusing substrate from an existing 10g tank with a similar algae problem, which broke. Sonce I still had fish to save, I started the cycle with the Marineland live bacteria pouch, which immediate drove a bacteria bloom similar to what I see after each water change. But it worked, as I was able to save my guppies, and quickly reach a good balance with zero ammonia and nitrates. So what is it, and how can I ger rid of it?