I have read that it is possible to get algae even though the CC is 0, you can pass the OCLT, and the water is clear. I have several light green spots on the pool plaster that are slowly becoming more prominent. I have been very careful to maintain the FC at 6-8 with a CYA of 40-50 with daily testing. SLAM is the next logical step. Guess I am surprised that some species of algae shows up anyway. I am brushing the pool surfaces regularly. These stains are quite difficult to remove, if at all, with vigorous brushing.
Is it possible to have iron stains that resemble algae? Since this pool is over twenty years old and my guardianship has been for less than one year, I don't know the history of what this pool has been subjected to.
Is this really algae?