I have a very stubborn case of Black algae (Cyanobacteria)that I haven’t been able to get rid of following the TFP recommendations. My 27,000 gal gunite pool is 20 years old and I suspect this Black algae has been in the pool for years and not treated. ( we lived out of country during this time). The mustard algae was taken care of but even after two weeks with an elevated FC level, daily wire brushing the Black Alage spots, and rubbing trichlor pucks over them there was very little noticeable change. I spent hours and hours besides mega money on all the liquid chlorine. I have read now that Black Algae can become resistance to Hypochlorous but that Chlorine Dioxide works. Has anyone used Chlorine Dioxide and if so where did you buy it from? Thank you