I've mentioned this in other posts, but I thought it would be good to discuss by itself. I have seen sudden drops in FC, which have been considered a red flag to check for phosphates. I have been told repeatedly on this forum that if FC levels are adequate, algae won't grow, so phosphates are more or less irrelevant. What I would like to know is why I see this sudden drop in pools with high phosphates.
This is most often an issue with salt systems. Chemistry will be fine for weeks, and out of nowhere the FC drops, sometimes turning pools cloudy or even green. All other chemistry seems to be within normal healty ranges. This isn't something that happens only in the peak of summer, it will occur from the time we start opening in the spring right on through the when we are closing in the fall. The one common denominator I keep finding is elevated phosphates.
Before you start asking, yes the salt levels are fine and the cells are generating. As I said they run fine for weeks or even months, and suddenly the FC drops. Once we remove the phosphate they return to working fine for weeks or months.
Please don't respond asking for specific chemistry measurements. I'm talking about a broad sweeping trend here. If I told you every time I see a pool with no chlorine in it, it turns cloudy, you wouldn't ask for specific measurements, you would simply say it needs chlorine. The trend I'm talking about is based on about 8 years of experience performing about 800 maintenance visits per season. These are not situations where CYA becomes elevated or anything like that. These are pools that are well balanced, crystal clear and sparkling that suddenly experience a drop in FC for no apparent reason.
Does phosphate interact with FC in some way to decrease its ability to sanitize? Does phosphate drive off chlorine resulting in low levels of FC? Is there anything about phosphates that can result in what I'm describing?