I have noticed this summer and last summer that my pool has some 'etching' or little holes that look like something has eaten out of it. Most of these pits or scars have some type of blackish algae in them that is very hard to get out. I don't have alot of this but there is a few spots around the pool that I can see when I get in the water with googles. I asume the pits were caused by something like chemical imbalance and then maybe black algae thought it was a good hiding place for itself. Am I right? What causes etching and pitting and how can I repair it and get rid of that black algae looking stuff?
My last set of numbers
ph 7.7 (I usually keep it from 7.4-7.8)