No Free Chlorine, High Phosphate, Mixed Cya Results

[canthony]

Hi, new user here.

My pool has become a bit of a disaster so I thought I'd bring my problems here.

Starting this spring, I started to get incessant algae blooms, both green and yellow. I followed my normal regimen of using yellow out and shocking with 73% cal hypo. Long story short, this did not work. I couldn't keep ahead of the algae and now my FC are consistently at zero. I have shocked a dozen times (switched to 10% liquid chlorine), using anywhere between 1 and 8 gallons of bleach. No matter how much I use, chlorine shows as zero by the next day. Shocking kills most of the algae but unless I put in a gallon every day it comes back immediately.

I tested my water, took it to a pool store, and had it tested by a pool pro. My CYA reading was 30, the pool pro's was 65, and the pool store's was 250! Needless to say, I don't trust the store as I use unstabilized chlorine products and minimal tabs. However, something is clearly going on here.

Is it just that there is so much algae in the pool that it is consuming my chlorine immediately? Or is there a more significant problem going on? Phosphates are over 1ppm, and I have begun treatment with Phosfree, but from what I have read on this forum that appears to be a waste of time.

More background: 20,000g, DE filter, new water a year or two ago. Last fall I used a large amount of Jack's Magic Copper & Scale remover (according to instructions). This removed my stains, but could be contributing to these problems.

Advice? Any would be appreciated. Thanks,

Craig

[chem geek]

Craig,

You first need to figure out your true pool water chemistry readings, especially for CYA. Please get a Taylor K-2006 or the TF-100 which has more volume of reagents so is comparably priced per test. If your CYA level is truly very high, then a partial drain/refill (or multiple ones) is in order.

Shocking is not a one-time event. You add more chlorine to maintain a high FC level that is roughly 40% of the CYA level. Initially, the chlorine will get consumed killing the algae, but if you don't add more, then the algae will just grow back. You need to get ahead of this curve by shocking to a high level and staying there until the algae is all killed and the pool is clear and there is <= 1 ppm FC overnight drop (with <= 0.5 ppm CC). You can see that to know your FC level when it's high you need a proper test kit and the ones I linked to both have a FAS-DPD chlorine test that can measure accurately up to 50 ppm FC.

Read Defeating Algae for more info.

By the way, I have over 3000 ppb phosphates in my pool shown here and here and prevent algae growth using a proper FC/CYA ratio using chlorinating liquid from my pool store, though bleach would work fine as well.

Richard

[canthony]

Richard, thanks for the quick response.

I believe the pool company I had out here was using a high end Taylor kit, but I have gone ahead and ordered a K2006 to double check. There isn't much point in me continuing to dump chlorine in if my CYA somehow actually is 250.

Until that gets here, I'll have to just try to keep things from getting too out of hand. BTW, what do we think the odds are that Jack's Stain remover is at all responsible for my problems?

Thanks,

Craig

[chem geek]

The fundamental cause of algae is not maintaining a Free Chlorine (FC) high enough relative to the Cyanuric Acid (CYA) level since chlorine can kill algae faster than it can grow regardless of nutrient conditions (i.e. phosphates and nitrates) since algae growth is also limited by sunlight and temperature. So if your CYA level is high, then that makes it difficult to prevent algae growth.

As for metal sequestrant products, some are phosphonates that can slowly degrade to orthophosphate and be a nutrient for algae, but I've got over 3000 ppb phosphates in my pool (and plenty of nitrates as well) but keep my 16,000 gallon pool shown here and here algae-free using chlorine alone. So while the Jack's product might have triggered the problem by providing more nutrients (maybe), it was not really the true source which was too low an FC/CYA ratio.

[canthony]

Alright, so my K2006 arrived and here are my test results:

FC: 0.2

CC: 0.0

PH: 7.5

CYA: 65

TA: 100

CH: 300

I consulted the pool calculator, and immediately removed 25% of my water. I then shocked with 10 gallons of 6% sodium hypochlorite. This should hopefully get me ahead of the curve and then I can begin regular maintenance tomorrow.

One interesting thing that I never thought about before: after shocking with so much chlorine, my pH increased to 8.2. Am I supposed to treat this immediately after shocking? I would imagine that adding acid at this point would simply react with and cancel out some of the chlorine.

[chem geek]

The pH will read falsely high at FC levels above 10 ppm though the pH does go up when you add the chlorine. When the FC level drops back down the pH will drop as well. So don't worry about that right now. In the future, if you are going to be shocking with more than 10 ppm FC or so, you can lower the pH level to 7.2 before doing the shocking.

10 gallons of 6% bleach in 20,000 gallons would add 31 ppm FC of chlorine -- is that more than you wanted to do? For 65 ppm CYA, the shock level is more like 26 ppm FC. Also, with your numbers (before any dilution) the bleach would have raised your pH to nearly 8.8 -- I think your test kit only reads up to 8.0.

Just remember that shocking is not a one-time event -- you need to maintain the shock level of chlorine until the pool clears, you have <= 0.5 ppm CC and you have <= 1 ppm FC drop overnight.

[canthony]

Actually, I was aiming for a mustard algae shock (as per poolcalculator.com) at my new cya level of around 50. Also figured it would be good to err a little high.

I measured my FC at around 10 am today, and it was 15. That's very encouraging to me, because previously my FC has always been zero in the morning. However, should I really expect a drop of less .5 overnight, even at such high concentrations (>31ppm)?

Pool looks great by the way, and the residual chlorine level tells me I'm on the right path. I haven't had my FC read above zero in months, except immediately after adding chlorine.

[chem geek]

I understand. As for the overnight FC drop, it should be <= 1 ppm, not 0.5 ppm. If that overnight is 8-12 hours, then that's an FC drop of 2-3 ppm over 24 hours with no sunlight. As for whether this is reasonable at such a high shock level, that's harder to say -- if it's <= 2 ppm FC overnight, then you could try lowering the FC into the 10-20 ppm range and see if the overnight drop is then <= 1 ppm FC. Usually, though, even at high shock levels people don't see much above a 1 ppm FC overnight drop, but that's usually at regular shock levels, not yellow/mustard algae shock levels.

[canthony]

By 6 pm yesterday, FC was back down to 5. To continue shocking, I then added 4 more gallons of 6% to bring FC back up to 15. It was 6.5 again this morning, although that was at 10am again so it's not a valid overnight test (alright, so I'm not an early riser). However, with a drop that large there doesn't seem to be any chance that I would have passed that test. Given those numbers, it's unlikely that I'll even be within acceptable ranges by this evening.

[chem geek]

Yes, you definitely have something in your pool consuming a lot of chlorine. If there is any visible debris, it is best to remove it. Check your skimmer and pump baskets as well as your filter (inspect and/or clean your DE filter). Unless you have a lot of visible algae or pollen, the amount of chlorine demand you are seeing is rather unusual (for overnight) and is usually associated with something going on chemically in the pool maybe leftover metal sequestrant or other products that are getting oxidized faster at shock levels. That can take a few days to a week before things settle down. This is one reason why one doesn't want to add anything to a pool that isn't necessary.

If you have debris that isn't getting filtered out, you can use a skimmer sock (or panty hose over the skimmer) to help remove the debris. Maintaining the shock level is important to ensure that you any algae faster than it can grow, but at this stage it sounds like you're now clearing out some of the chemicals you previously added.

[canthony]

Well, I really thought I had it. I maintained the shock level at 15 all day long and opened up and cleaned out the filter. At the end of the day, the FC level seemed to have stabilized. I went to bed with it at 17.5.

But I checked again this morning at 10 am and, lo and behold: FC 9.5.

So it continues. I added 192 oz of 6% to bring it back above 15 and I'll try to leave it there all day again.

[canthony]

Success, or so it appears. At midnight last night, FC was 13.5. This morning, it read: 13.5. No measurable drop overnight (of course, this was using the .5ppm accuracy K2006 test).

By the way, my pool looks amazing. For the three years I have lived here, I have had constant mustard algae growth on the walls of my pools and increasing trouble with green algae. I have spent hundreds of dollars in the past few months trying to fix this, and in the end it has been completely resolved using about $30 worth of bleach.

Thanks to everyone for the help.

[wsommariva]

How about your phosphate level?

[waterbear]

How about your phosphate level?

Phosphates are generally a non issue, despite all the hype. Realize that phosphate removers are a GREAT money maker for pool stores (I've worked in pool stores and we were always told to push the phosphate removers! ( Personally I didn't like pushing a product that was not really needed and that generally cause the pool to go cloudy for up to a week.)

[wsommariva]

I was indeed a little suspicious when I heard that phosphates were the "new" word in pool care.

[waterbear]

I was indeed a little suspicious when I heard that phosphates were the "new" word in pool care.

No, just the new cash cow.

[2n3055]

Waterbear, what made you leave the pool store business?

[waterbear]

Waterbear, what made you leave the pool store business?

moved on to greener pastures.