FC dropping almost 3ppm overnight with CYA sitting right at 40

Started by Alan Pierce · · 2 replies
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#1
Been keeping FC at 6ppm and CYA tested at 40 with the TF-100, so that should hold pretty steady overnight with no sun eating at it. Instead I'm losing close to 3ppm between evening and morning test, three days running now. No algae that I can see, water's clear, brushed everything Sunday. Phosphates aren't something I've ever tested for, wondering if that's worth chasing or if I'm overthinking a testing error on my end. Reagents aren't expired, I checked. Only other thing going on is we had a hot stretch here in Marietta, mid 90s, so I know that speeds up UV loss some, but this feels like more than heat alone. Anybody seen a drop like this land on something besides the obvious sunlight and algae answers before I start throwing shock at a problem I don't understand yet.

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#2
Not gonna solve it for you but that overnight drop with no sun caught my eye, our system runs the SWG less at night obviously but even with that I dont think ive ever lost more than about a ppm just sitting overnight with CYA around 30. 3ppm with no algae and clear water sounds like something is actually consuming it, not just fading off on its own.

One thing I didnt see mentioned, do you have anything organic getting introduced at night, we went through a stretch where our number was off and it turned out to be something dumb like a cover corner dipping in water each evening. Not saying thats it for you, just throwing out that the timing of when its happening might matter as much as the amount.
#3
Something's demanding that chlorine, CYA at 40 doesn't burn off 3ppm overnight sitting still. Before phosphates, check for a leak pulling in something organic, or just retest with a fresh sample straight from the pool, not from a bucket that sat. Bather load or a dead critter in the skimmer basket you didn't notice would do this too.