I'm a fairly new pool owner (built our pool in May 2015). I was instructed by our pool company to clean our filter cartridges every 3-4 months. I finally got to it yesterday. Rinsed the cartridges with the aqua comb, then soaked them in dishwashing liquid mixed with water overnight. Two of the cartridges still have a brown stain on one side after soaking and rinsing. Do I need to do anything else? The pool company told me to soak them in water and muriatic acid every time I clean them but based on everything I've read it sounds like that might be harmful.
Does anyone know what these stains are from and how to clean them? Please see a picture of stain attached.
Thanks so much!