If you're talking about the red stuff....that would be Cyanobacteria! If you do a search for it, you can find all kinds of threads regarding it as well as Chemiclean, a product used to treat it (if you choose that route).
If you're talking about the red stuff....that would be Cyanobacteria! If you do a search for it, you can find all kinds of threads regarding it as well as Chemiclean, a product used to treat it (if you choose that route).