Came back from a 3 day vacation and there was thin layer of algae evenly throughout entire pool surface. My cya was about 40 so I brought the FC up to 16-20 and kept it there all day. I brushed the entire pool twice. The FC level only dropped a little once and I immediately replenished it. My pump and vacuum have been running the entire time. The same night i checked the FC was at 17 right after sunset and right before sunrise it was exactly the same. So everything is passing like i'm out of the woods. My only concern is that when i brushed my pool this morning again i can still see algae coming up.
Is this old algae that is dead and just needs to be filtered out? Or do I still have algae?
Should I keep the FC at shock levels for another day?
And after I let the FC levels fall, at what level is safe for swimming? In other words, with a CYA of 40, what is the highest level of FC that is safe for swimming?