How can I have algae when maintaining at 7ppm

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I am having exactly the same issue. I have CYA little bit less than 40 and maintain the FC on 7ppm every night. It has been around 4.5/5 when I run tests so I just add some to make it 7.
But algae grows every a few days and I have to SLAM every other week to bring it to crystal clear.
Still no clue...
 
Ok, So just got back from UK last night. Was away about 2 weeks.

I added bleach before I went and was at FC 40 on 27th June.

Tested last night at 5.30pm and was at FC 7 - with SWG running at 62% on normal 6 hr cycle.
1. Is that a pretty big drop in 2 weeks, shouldn't the SWG hold it the level I left it

I tested again this morning at 7.30 am and was FC 6.5 - so that is a definite pass - only lost .5 even with sun still out for 2 hrs before overnight.

At 1 pm today, Fc was 5.5 , so I LOST 1 FC since this morning. I found that strange so tested CYA and it was about 45 !

I have no idea what is eating my CYA. It was at 60 on 19th June, I added enough to raise it to 80 the day I left (on 27th June) and come back two weeks later to find it at 45 ...makes no sense - any ideas? I do not have a leak as water level drop was normal (about 4 inches) for the 2 weeks I was away.

Anyway , I have now turned SWG up to 90% until I can get some more CYA. I think I will switch back to liquid as it seems way better.

So, upside is my pool survived me being two weeks away !

Downside, nanobots are eating my CYA.

thx
 
Elevated FC way above target is consumed by UV at a greater rate.

CYA is oxidized at a greater rate at high FC and UV. So that loss is possible. Though high. Could the pool have overflowed some while you were gone due to rain?
 
Ok, So that would make sense, if you can a loss, and I did have very high FC (40) and obviously high UV. There was not enough rain to flood pool while away.
I turned SWG to 90% at 1.30 pm yesterday after the 5.5 FC reading, then at 10.30 am this morning it was at FC 8.

As I am happy that I am algae free, I will raise the CYA back to 80, and drop the SWG back to 60%.

I made the mistake before of raising CYA to 80-90 and then finding I has to SLAM !! So I always do an overnight now, before ever raising CYA . \

thx
 
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