Hi all! Any ideas what would cause tea colored water to turn black? I put activated carbon in my waterfall to rid the pond of the tea color and now it's black! I rinsed the carbon until the water ran clear and placed it in a mesh bag. I test the water weekly. Today ammonia and nitrite tested 0. Phosphate tested 0.5 and PH tested 7.5. Could it be algea? I have some clear tubing circulating the water and there appears to be some black spots on the inside of it. Any suggestions on clearing this up would be great. PS. 2000 gallon pond, 4000 gph pump,4000 gph tetra UV bio filter, and 10 small fish.