So a month ago, I posted a thread sharing how much CL I put in my pool daily and asked how much others are doing so. Was cruising on 2-3 ppm or so, and I thought for my pool it was exactly a quart a day.
But what I didn't notice was that my CL demand was slightly more than a quart, maybe a quart and a half on a heavy use/sunny day... so my chlorine kept creeping down. Was at 6-8 ppm, then 5-7 a few days later, then 4-6 another few days later. But the change was so gradual that I didn't even notice... until today, when I tested 2.5 FC (CYA at 50). Whoops! Dumped a gallon in there to bring it back up to around 8.
Time for an OCLT to check to see if any algae found a new home, but I'm hoping bringing FC to 8 nipped it in the bud! This TFP method is really easy, but you have to be on top of it every day! I can understand why people go the SWG route.