CYA really low. how is this possible?

[diasurfer]

I took over my pool in January of 2013 and installed a SWG. With help from this forum, I got everything set up and had a successful first year (am having some black algae show up recently).

Lately I've noticed FC levels are dropping faster and figured I need to clean and/or replace my SWG cell. But since it's been awhile, I decided to go ahead and do a full chem test.

FC 2.5
CC 0.0
pH 8.0+ (lowered again to 7.2, my pH always drifts high due to SWG).
TA 55
CH 375
CYA 0 ????

The CYA test is what really shocked me. I did this test repeatedly last year while trying to dial in my CYA to around 60, and am confident I am doing it right. I'm filling the test tube all the way to top and am barely noticing any clouding of the black dot.

My salt seems to fall relatively quickly too. We get a lot of rain and evaporation, but I didn't think these things were supposed to change CYA (or salinity). Splash out shouldn't be a major issue in my pool as most of the swimmers are less than 5 years old.

Is a leak the only explanation? I might have added a little water two or three times last year, but generally evaporation and rain balanced each other out.

My pool test kit (Taylor K-2006) are a little over a year old and have been kept indoors in the dark. Is it possible CYA reagent is bad?

 

[JasonLion]

It is fairly common for CYA to vanish over the winter. There is no equivalent for salt, salt only goes down via water replacement. So if salt and CYA both go down by about the same percentage then there is probably a leak. But if CYA goes down over the winter and salt remains more or less like it was, then it is the CYA vanishing thing.

 

[diasurfer]

Wow. Somehow I missed the CYA vanishing thing in my reading last year. I was under the impression that since the only way to lower CYA (intentionally) is to replace water, then the stuff lasts forever. I was wrong! I suppose the best strategy is to get CYA up and start paying close attention to the salt.

At the same time, I'm going to scrape my black algae and rub with trichlor tablet. If I leave the tablets sitting on the spots, they will increase CYA level as well. Would this be a significant amount?

 

[Isaac-1]

In an operating well balanced pool CYA will VERY SLOWLY break down (a few percent per month), but usually way too slow for effective reduction, and often at a rate that is masked by splash out, etc. However ina pool that is shut down for the winter a type of bacterial conversion can happen that converts the CYA to Ammonia, the exact process of this conversion is not well understood, and it does not always happen.

 

[chem geek]

If you are unlucky, the bacterial conversion of CYA ends in ammonia resulting in a HUGE chlorine demand upon opening. If you are lucky, the conversion goes all the way to nitrogen gas in which case one doesn't have the large chlorine demand upon opening and the CYA just magically seems to disappear.

 

[diasurfer]

Well my pool was never really "closed" in the sense of pools in cold climates. We didn't swim in it very often in January though! I never noticed anything really unusual over the winter as far as chlorine demand. I think I'm not significantly out-of-balance but am off enough to say what is really happening (or happened). I'm going to get salt up, add CYA, scrape the black algae with trichlor tabs, clean the SWG, and replace the filter and see what happens. Spring to-do list.

 

[duraleigh]

I would tackle that black algae more aggressively.

With CYA around 30 ppm (temporarily) I would bring the FC up to around 10-12 ppm and HOLD IT THERE.

Next, I would scrub the black algae every day with a stainless Steel brush until I saw no visible sign of it.

Last, return CYA and FC to normal but keep a close eye for any visible black algae and KEEP THAT CHLORINE UP.

 

[anonapersona]

How much rain did you get over the winter? Enough rain to cause the pool to spill over an edge or an overflow grate will cause CYA and salt to drop, as well as CH and TA.

 

[diasurfer]

duraleigh, that's good advice. Low CYA is my opportunity to wipe out the black algae. How long should I keep FC at 10-12 ppm?

 

[duraleigh]

Some folks think elevating your FC above normal FOREVER is a good plan once you get black algae.

Personally, I would keep FC elevated until I saw no visible sign of it remaining and then let my FC drift back down to normal level but still keeping a watchful eye on any sign of the black algae reappearing.

 

[chem geek]

With black algae it is VERY important to scrape off the heads to allow higher chlorine levels to get to the roots of the algae. Use a wire pool brush (for a plaster pool) to remove the waxy surface layer of the black algae.

 

[HouTex]

Not sure how much black algae you are talking about. Just about every 6-8 months I get some black algae along the southern side of my pool on the tile grout at the water line. It's along the wall of the spa and the tanning deck. Both areas don't get great circulation. I just follow the advice here whenever it gets noticeable and it's easily removed. I don't worry about it because my pool is otherwise crystal clear and it's never even been cloudy. I do not increase my daily chlorine level. Scraping pretty much does the trick and it takes about 15 minutes to remove it.