Chemicals Balanced But Starting to Grow Algae--PLEASE HELP!

[aesweeps]

Hi. I have a 25,000 gallon in-ground vinyl liner pool. I replaced the liner just recently in April 2014. Water has been free and clear of algae until the past couple of weeks. I've been finding small patches of it moslty along the wall and in corners. I did a thorough brushing around fixtures and some came from around the steps also. I usually shock the pool once per week, or right after I find and brush the algae. Chemicals seem to be somewhat balanced and are listed below. Water is sparkly with the exception of the small algae patches tht I'm finding at least every couple days or so. I use the Taylor k-2005 test kit. Several weeks ago I was having problems with the alkalinity being too high, at about 150ppm, but then it went down to 120 and now 100. Any reasons why my pool is starting to grow algae? and suggestions on how to fix my problem. Thank you so much in advance. Anna

FC-2
TC-3
pH-7.6
Alk-100
Calcium Hardness-100
CYA-62

 

[JasonLion]

Welcome to TFP!

Your FC level is way too low for your CYA level. With CYA at 62, you need to maintain FC between 5 and 9, and never below 5.

If you can't quickly get rid of the algae, you will need to SLAM the pool, see instructions in the How To section of Pool School.

 

[Divin Dave]

Unfortunately, the pool stores do not understand the FC to CYA relationship. Even though it has been known since 1974.

http://www.troublefreepool.com/content/128-chlorine-cya-chart-slam-shock

 

[Richard320]

Welcome!

Ladders and stairs are notorious hiding places for algae. And once it's in there, the circulation is poor, so the chlorine doesn't get in and kill it. And then it starts its insidious expansion out into the pool. As it grows, it uses up the chlorine, which allows more algae to grow which uses even more chlorine, which allows even more algae to grow and on and on until the pool is green and nearly thick enough to walk across the surface.

So kill it. Kill it all. That's what we call a SLAM. Shock Level and Maintain. Shock level means it's killing it at a much faster rate than it can reproduce, and maintain means holding it there for as long as it takes to kill off every last algae spore.

If the water is generally clear, it can be over within a day or two if you're diligent. Maybe even less is you're obsessive.

Just follow the instructions in the SLAM article exactly. In your case, a little acid to lower pH to 7.2ish, and while that's mixing, pull any steps and ladders and start scrubbing them. And then hit it hard with bleach - shock level is 24 with your CYA - brush 'til your shoulders ache, retest, hit it again to 24 and on and on. If you can watch it all day, you might even be ready to do the overnight loss test tonight!

And after that, just take care not to let FC dip below 5 and you're golden for the rest of the summer. You'll never need the weekly shock if you never let algae start again, and that means maintaining the chlorine level. If you're using pucks to chlorinate, be aware that they raise the CYA level and then you'll need to raise the minimum FC you have to maintain to stay ahead of any algae.

 

[anonapersona]

And no more products that contain CYA!

Not until that falls to 30 ppm, then you can choose between straight CYA and other products that include CYA with chlorine.

Until then, straight chlorine for you. Bleach that is, or Liquid Pool Shock.