I use them for the rabbits, chickens, and goats but have never had that. I have had green algae in the summer. It may be the water supply with the heater helping it to grow. Why, in home systems that utilize shallow wells with bladder tanks the "black scum" is fairly common adhering to the rubber liner. In some cases, later on, when you open high volume faucets, like a tub, it may come out and is gross looking. Jacuzzi type tubs are notorious for this as the water sets in the pipes and black scum forms. Check your water system.