It seems that you were correct and the stain was in fact an algae of some type as after the first night with the Chlorine at 15 most of the stain disappeared and the filter pressure had gone up from filtering the dead algae, I'm guessing brown or black. After getting my pH to stabilize, I increased my Chlorine to 15 by adding the 9 gallons of Bleach. I've had trouble keeping the Chlorine at the 15 ppm level, I"m almost thru the third day and I've added an additional 7 gallons of Bleach to try to maintain that level. I will say that about 85% of the stains have been removed and I'm hoping that if I keep the level at a shock that remaining stain will be removed as well. How long does it generally take to kill an algae of this type?