I recently noticed small black spots on my gunite. And they grow. From what I've read it seems like black algae. They are very tough to brush away (metal brush). I've tried shocking the pool to no avail. Is there a sure fire way of ridding my pool of this stuff. My chemicals are in balance. 10k gunite with SWG in Texas. I added the salt generator a little over a year ago because green algae was driving me crazy. any help would be appreciated.
Thanks