Algae with enough FC (I think!)

[aenman]

Thank you all for all the info you've given me, and thanks for convincing me to switch from biguanides before last year.

On chlorine now, 2nd full season. I'm a new poster and sorry to create a new post similar to what's been out there before, but I'm just stumped.

FC 4.0
CC 0
PH 7.2
TA 90
CH 120
CYA 25

Pool is currently 82F. I noticed some algae last Sunday, primarily growing in the little wrinkle valleys of the vinyl bottom and in the areas of the ladder where there's not much circulation. I had never let my FC drop below 2.5. I know my CYA is a little low, but I test for chlorine every day and keep the pool covered with a solar cover when not in use - with the solar cover on I barely lose any FC even during the day. We typically only use the pool during the weekends, so it is covered all week, and it is only two of us for a low bather load. Before last Sunday, the pool had been covered with a solar cover for 2.5 weeks since it had gotten a little cool (air temps in the low 70s, I did not brush or vacuum during this time). My pump runs 14+ hrs per day. FC has never dropped below 2.5; typically around 3-4 since I opened in spring. Water is perfectly clear.

Not sure what I did wrong to allow the algae to get started - as I mentioned FC was always above 2.5. Also, now that it has begun, I believe I have to SLAM to get rid of it (can't just brush regularly and keep the FC around 3-4, right)? Any advice is welcomed, especially what I might do differently to avoid letting it get started again.

Thanks so much for the helpful posts and info on this site!!

 

[northrn]

Visible algae does equal a SLAM.

Have you checked an overnight loss test to see if you're dealing with organics vs dust/debris?

 

[aenman]

Thanks for the response!

Overnight FC loss is much less than 0.5 per night, which is one of the reasons I'm a little surprised. That is with the solar cover on - I assume that skews the OCLT somehow - is the <1 ppm overnight recommendation with a blanket on or not, or does it matter?

I want to believe it's just dust, but the stains were pretty green and organic looking (shape and complete disappearance when brushed).

 

[northrn]

According to the OCLT process, you should raise your chlorine level higher than 2.5 to check for loss. I don't believe the blanket makes a difference unless it's the source of algae.

You're sounding like a SLAM. Caught early, it shouldn't take long. Good luck!