We mentioned earlier that your CYA was also low. A low CYA means less protection from the sun for your chlorine (FC). Were you able to add more stabilizer yet? That could explain why your FC is falling so quickly during the day. The TX sun is getting hot.
So no stabilizer yet. With my struggles to get any type of actual reading on FC, I've been focusing on getting that into the whole number level. I also understand that the higher the CYA, the more chlorine it takes to SLAM which is where I am now.
We started the season off with algae, though whether it is green or black is questionable at this point. With how it appeared, Leslie's diagnosed it as black. When we spent 3 hours brushing the pool last weekend, I'm inclined to think otherwise. Our pool has not been green and we had the classic black spots on our walls, though it was relatively minimal. After getting sucked into Leslie's $65 bottle of "black algae killer" & other instructions and still not getting anywhere is when I stepped into this picture. We brushed endlessly and it looked much better. Started adding the shock and researching when I found TFP. And here we are, lol.
I can run a FC test with my eyes closed now. After adding the 3rd gallon of 7% bleach this morning and seeing a 1.5 FC, I was thrilled. Tested again after picking up liquid chlorine this afternoon and was back to .5. So we backwashed, added 2.5 gallons of 10% and am now sitting at 11 about an hour after adding. We'll see how that goes. That was about 30 minutes ago.
Will be picking up the stabilizer tomorrow and definitely want to up my CYA, but am hesitant due to increased need for chlorine. Seems to be a tie. Sun burns up chlorine too fast due to low CYA (still hovering around 30) resulting in higher need for chlorine or increase CYA and need more chlorine to SLAM.
I opted to start the SLAM process as I'm not convinced our algae problems ever truly went away despite the pretty blue water. We couldn't get our levels high enough to kill it all. I've got 2 more gallons of 10% and can get more. My local Walmart has about 50 more jugs on the shelf. Will be a loooonnng weekend, and not in a good way, lol. But I'm cautiously optimistic since finding TFP!