Even us "experts" have issues from time to time, we are human and make mistakes. I'll share my tale of woe.
Black Algae. It happens. Diamond Brite surface, gives a nice place for it to grab on to. So here in Florida, water temps get close to 90 degrees in the summer, uncovered pool. Lots of rain. I know better, but I don't test CYA every week. I have a SWCG, no worries. WRONG.
See in the summer, high usage, lots of splash out, and rain. You may tend to forget, chemical levels, CYA, Salt, and CH levels will go down. It's the nature of the beast. Those aren't in the water, or rain and, will go down. I test FC/PH every few days. We'll my SWCG wasn't keeping up, FC level was going down, and the Black Algae was exploding! I was brushing, added chlorine.
Then it dawned on me.
Run the full set of test! My CYA level was low. Added 5 lbs of CYA. FC level is increasing! Brush, Brush, Brush. Black Algae is back under control! I'm winning the battle.
Maintain a higher FC level.
All is well, but I still hate Black Algae.
Lesson learned. Don't skimp on testing. Even when you know your pool.