High FC vs Phosphate testing

[FormerBromineUser]

TCP007 has been talking to someone as he has 23 posts, but only 3 show up to us NIP's. Would be interesting to see more history, especially to clear up the 5 FC question. Good luck with your closing, 007!

[BigDave]

FormerBromineUser makes a good point, this thread seems to be missing context. I found TCP007's earlier thread in the getting started forum and re-read it. Here's a quick review as I understand the situation. Please correct as necessary.

Before coming to PoolForum, TCP007 was having a bad season this year with recurrent algae. TCP007 read some of the posts here and decided to shock with liquid chlorine.
TCP007's pool had little or no CYA when first tested with a K-2006 and after the bleach(LC) shock FC and CC were quite high.
Granular CYA was added and the rate of FC loss slowed. No more chlorine of any source was added before this thread was started.
As the shock chlorine was eventually consumed, the pool developed algae once again.
TCP007, being the resourcefull sort, read the blue book that comes with the K-2006 (we usually tell people to throw it away) and found that phosphates can contribute to algae growth.
TCP007 sought to mitigate the influence of phosphates with a phosphate remover.

So, back to basics.
Pools need sanitizer (chlorine) to remain clean and safe.
Pools need stabilizer (CYA) to protect chlorine from sun exposure.
Pool water must have a pH between 7 and 8 for swimmer's comfort and to protect the pool and equipment from damage.
Concrete / plaster pools must have calcium in the water to help prevent the water from taking it from the pool walls.
Pool water needs a balance between calcium hardness, total alkalinity, and pH to prevent calcium scaling on one hand and calcium leeching on the other.

Earlier in this thread I think I wrote about identifying mustard algae; Do you have it?
Please provide a complete set of readings from your K-2006.

If FC is less than 10% of CYA, get some chlorine in there ASAP. We like bleach best as it has fewest side efects.

[chem geek]

BigDave: No water change, but yes... I stopped adding granular dichlor since I super-chlorinated with liquid chlorine in late July. A mod here recommended I stop adding sanitizer until my FC was at or below 5ppm. I actually added my floater back to the pool in mid August because I was worried about not having any sanitizer whatsoever. The floater holds about 1-lb of 1" trichlor tabs that typically dissolve in about 1 week. But that's the only sanitizer I have added since late July until about 2 weeks ago. I test my water every Saturday, and my FC was still at 7ppm the week prior to this last, minor outbreak.

1 pound of Trichlor tabs in 35,000 gallons would only be 3.1 ppm FC of chlorine (and 1.9 ppm CYA) so if that's the only chlorine you've added from July until 2 weeks ago, that wasn't nearly enough since that's only 0.44 ppm FC per day. More usual daily chlorine loss is 2 ppm per day. Do you have a solar cover? Is your water temperature cold?

I suspect that your chlorine level was low for an extended period of time and that allowed algae to grow. Once algae is established, however, it takes higher levels to kill it which is why we recommend shocking the pool to higher levels to kill off the algae.