Didn't think FC could vanish this fast just sitting overnight

Started by Ray Castillo · · 4 replies
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#1
Tested at midnight before bed, 5 ppm FC, felt good about it. Tested again 6am before work, down to 1.5. No sun touched that water for six hours straight. Figured it was bather load from the day before still eating chlorine, or the pool just running hot overnight even in October. Checked CYA and it's sitting around 40 so that's not it either. Only thing I can think is heat alone keeps demand up even in the dark out here, water was still 88 at midnight. Anyone else seeing overnight loss like that or is something else going on with mine, ordered more test strips just to rule out my eyes doing something weird with the color.

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#2
Ray, before anything else, what's actually driving that 3.5 ppm drop overnight, I'd want to know if you're running any kind of algaecide or if there's an ammonia source, because CYA at 40 ppm doesn't burn off chlorine like that on its own, sun or no sun. Also worth asking, are you on liquid chlorine or a tab feeder, since a leftover trichlor puck sitting in a skimmer basket can throw your readings around in ways that look like loss but are really something else eating it or just measurement weirdness. And test strips are honestly not precise enough to chase a number this specific, they're fine for a ballpark but not for pinning down a drop like this.
#3
Alan asking about ammonia source made me think, do you have any fertilizer runoff nearby or did anyone maybe not shower before swimming that day, thats usually my first guess when chlorine just disappears like that. Also is this a liquid test kit or are you eyeballing strips right now, cause a 3.5 drop is the kind of thing id want to confirm with drops before chasing ghosts, strips have fooled me before.
#4
3.5 ppm overnight is a lot for just heat, I'd lean toward what Dana said about somebody not showering first or algaecide being in the mix. Never seen my chlorine drop like that just sitting there in the dark, and mine sits at 88-90 plenty of nights in July and August here in Mobile. I run trichlor tabs same as always, test at night and again in the morning most of the summer, and I might lose half a point, not three and a half. If it were me I'd get an actual liquid test kit before chasing this any further, strips have led me wrong before too.
#5
Earl's right to push on the test kit, strips are garbage for pinning down a specific drop like this. Mine's a Taylor K-2006, it's the only one I trust because with fill water over 400 ppm calcium I need something that actually shows a clean endpoint instead of guessing at a color on a strip. Get the real kit before you chase ammonia or algaecide theories any further, you might find that 3.5 drop is half real and half bad measurement.