FC won't hold overnight, pH steady, algaecide already ruled out

Started by tampapoolguy · · 3 replies
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#1
Got a weird one, and it's my own pool this time not a customer. FC at dusk reads 4, next morning it's down to 1 or lower, every single day this week. pH holding right at 7.6, TA around 90, CYA I just tested at 60 so it's not a CYA problem masking things. No algae visible anywhere, brushed every surface twice looking for it. No leaves, no obvious organic load. Salt is fine, cell output looks normal.

Only thing I can think of is something's off with a bather load I'm not accounting for, but nobody's been in it. Checked for phosphates just to rule that out too, low.

Seen it a hundred times with other people's pools and usually there's some hidden algae bloom or a leak pulling in stuff, but I'm stumped on my own. Anyone dealt with FC vanishing overnight with chemistry otherwise boring like this.

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#2
This is a bit above my pay grade compared to what you deal with professionally, but one thing I didn't see mentioned, is the pool covered at night or is it open to the sky? Gene and I lost chlorine fast one summer and it turned out to just be sun exposure hours plus that CYA of yours at 60 not doing quite as much protecting as folks assume once it's evening and the water's still warm. Also, any chance something new is shading part of the pool during the day, or did you change anything with the pump run schedule this week.
#3
Marlene, the sun exposure angle doesn't hold up for an overnight drop though, that's the one thing UV can't touch since the sun's not out. FC loss from UV happens during daylight hours, that's exactly what CYA at 60 ppm is protecting against, so whatever's eating his chlorine between dusk and dawn has nothing to do with sunlight. Tampapoolguy, with pH steady, TA fine, phosphates low and no visible algae, I'd start looking at something biological you can't see yet, a thin biofilm on a return fitting, ladder, or skimmer basket that hasn't bloomed into visible algae but is still consuming chlorine as fast as you add it. I had something similar years back and it turned out to be slime built up inside a threaded fitting on my suction line, invisible until I pulled it apart. Also worth double checking your salt cell isn't fooling you on output, since "looks normal" on a display doesn't always match actual production, an overnight FC test right after adding chlorine versus one at 2am would tell you whether it's a steady bleed or something sudden.
#4
Alan, the biofilm theory feels like a stretch for a 4 to