FC eating itself faster this week, nothing obvious changed

Started by Priya Raman · · 3 replies
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#1
Normally I dose to 4.5 FC and it holds for about three days before I need to touch it again (screen cage, so debris isn't the variable). This week it's gone from 4.5 to under 1 in about 36 hours, twice in a row. pH steady at 7.6, TA 80, CYA 45, same as it's been all season per my log. No rain, no extra swimmers, cage is intact as far as I can see. Ondilo's showing the same crash between my manual tests so it's not a bad reagent thing.

Only thing I can think of is something organic got in that I'm not seeing, or a hidden algae bloom starting somewhere I can't spot it, sun shelf maybe. Anyone had a mystery demand spike like this resolve on its own or did you always find a cause. Not ready to shock yet since FC still reads something, just annoyed I can't explain it.

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#2
Everything else steady but chlorine tanking that fast usually means algae starting somewhere hidden, sun shelf or steps are exactly where mine hid it last time before I could even see green. I couldnt spot it either until I brushed the shelf hard and the water clouded up a bit, so worth a good brush down there before you write it off as a mystery.
#3
I'd push back a little on jumping straight to hidden algae, Dana, mostly because Priya's CYA is 45 and FC is still reading something above zero at the 36 hour mark rather than crashing to nothing. Early algae demand tends to accelerate, each day worse than the last as it establishes, not hold a consistent 3.5 ppm drop twice in a row like clockwork. That repeatability is what's nagging at me, it reads more like a fixed input than a growing biological demand.

Priya, before brushing the sun shelf, I'd want to know if anything changed with what's going into that pool even if it seems unrelated, sunscreen residue from more pool time even without extra swimmers, a leaf or two of something that decomposed fast in the heat, even a saltwater fish tank dump or fertilizer runoff if you're near a lawn that got treated. CYA at 45 protects your chlorine fine under normal UV loss, it doesn't explain a sudden organic load appearing out of nowhere. Since it's happened twice identically, I'd also double check your FC and CYA numbers again with a fresh test just to rule out a testing consistency issue before hunting for algae you can't see, because if the ratio's really behaving the same way both times that's a clue in itself, not just an annoyance.
#4
Alan's onto something with that repeatability, twice in a row the same drop is odd for algae which usually just gets worse each day like he said. One thing nobody's mentioned yet, any chance somebody's using a different sunscreen or bug spray around that pool lately. Some of that stuff eats chlorine fast and if its the same one or two people using it before their swim you'd get that same consistent hit each time. Wouldn't show up as green anywhere, wouldn't need extra swimmers either, just the same routine with a new product in it. Worth asking around your house before you go tearing that sun shelf apart with a brush.