My gunite pool is about 18 years oil. Last summer I started getting a few small black spots on the bottom of the pool. This summer I have many more, no matter how hard I brush I cannot get rid of them. Algae treatments do not seem to help. My husband used to do all the pool work, however since he passed 3 years ago I have tried to learn it all as I cannot afford a pool service. This looks terrible and is so frustrating! I can take care of it if I know what is wrong. I am wondering if it is the gunite wearing off? Or maybe the algae is very bad and stubborn and I am not strong enuf to brush it off. Any help?? I am in Texas and I know as it keeps getting hot I will have even more problems. Recent pool water tests show I am doing ok except the phospates were up, which I am working on, and the cyanic acid was about 100.