Persistent algae

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Freya

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Jul 8, 2013
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Algae seems a constant in my pool (this is my third season with this pool, which is about 20 years old). I shock seemingly killing it all but it comes back within a few days. I keep FC around 3ppm and top up as soon as it drops below 2ppm but this doesn't seem to stop algae building up. Every time I brush the sides and bottom (weekly) a green film is visible so I shock again and it is fine for a few days. I've tried extreme shocking (>30ppm FC) for a week, flocing and vacuuming to waste but again within a few days of normal operating FC levels the green film is back. I replaced the sand in my filter this year but no change and I backwash regularly (every 2 weeks or if pressure goes up 5psi).

All other chemicals seems fine e.g. currently I have:
FC 2.25ppm
pH 7.4
CC 150ppm
TA 90ppm
CYA 50ppm

The water is clear right now and I'll add a little CC today and some liquid chlorine tonight but I'm sure tomorrow I'll see some green when I brush as it has been fine for 3 days now.

Any ideas why the algae is coming back so quickly and virulently? Should I use algaecide? Should I run with FC higher than recommended, say 5ppm? Should I switch to salt or add borates? The pool only gets light use mostly on the weekends.
 
Welcome to TFP!

The reason you constantly have algae is because you are keeping FC too low. With CYA at 50, you want FC between 4 and 8, and never below 4. Where ever you got the idea that FC at 2 or 3 is acceptable is just wrong.

Also, when you wrote CC I think you actually meant CH (calcium).
 
Thanks - OK I'll maintain FC above 4ppm - hopefully this will cure the problem. I must have read 2-3ppm as being the recommended range for swimming somewhere - I presume from a skin/hair/smell/taste perspective? Is it better to allow CYA to drop so FC can be effective at a lower concentration? How low can I let CYA go - and is the trade off a faster dissipation of FC? Are there any risks to swimming with high chlorine?

Yes I meant hardness (from adding calcium chloride - which is what I meant by CC).

I presume I should go through another shock treatment first?
 
There is no reason to try and lower the FC level. With FC between 4 and 8 and CYA around 50 you are right in the ideal zone. The rules about keeping FC low date back to before anyone used CYA or even understood how it worked, and are just plain wrong for modern outdoor pools that use CYA.

Yes, time to shock.
 
Yes I have a test kit but the numbers I posted are from the pool shop where I get my liquid chlorine (just stocked up). I haven't found much difference in the pool shop numbers to mine - there always is a difference but it seems random...no idea which is right.

Just brushed and netted and as expected I got a light green trail - I've started the shock process, no point waiting till tonight.
 
It seems you may have have the wrong idea on how to shock a pool. You may be clearing up (to your eyes) the problem with a one-time addition of a product from the pool store. Shocking is a process, not a product.
pool-school/shocking_your_pool

Until you follow the shock process through completion, you'll continue to have the problems, even at the proper FC level.
 
RobbieH said:
It seems you may have have the wrong idea on how to shock a pool. You may be clearing up (to your eyes) the problem with a one-time addition of a product from the pool store. Shocking is a process, not a product.
pool-school/shocking_your_pool

Until you follow the shock process through completion, you'll continue to have the problems, even at the proper FC level.
Click to expand...

Thanks - what do you think I have wrong about how to shock a pool?
 

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Your first shock comment sounded like you made a single addition overnight. Then you said you did extreme shock for a week, but did not mention if you passed the three criteria to know it was complete: 1. Pass the overnight chlorine loss test, 2. CC of .5 or less, 3. water is clear.
 
JasonLion said:
Welcome to TFP!

Where ever you got the idea that FC at 2 or 3 is acceptable is just wrong.
Click to expand...

Checked the CDC website and they sort of recommend FC in the 1-3ppm range. Although they do say disinfectant times are longer in the presence of CYA which implies using higher chlorine concentration but they don't actually suggest raising FC. The health risks seem to be from chloramines (combined chlorine?) and not from high FC - which paradoxically they say to treat by RAISING FC - which I thought was interesting (and also because water companies often treat drinking water with chloramines). pH seems key to disinfectant potency.

Anyway, I'm sure you know all this but at least I know where I could have got the 1-3ppm range from!!
 
RobbieH said:
Your first shock comment sounded like you made a single addition overnight. Then you said you did extreme shock for a week, but did not mention if you passed the three criteria to know it was complete: 1. Pass the overnight chlorine loss test, 2. CC of .5 or less, 3. water is clear.
Click to expand...

My first comment didn't sound like a single addition overnight - you are simply guessing (wrongly). But it is good to remind people of the process so thanks.

By the way, is the test for CC of 0.5 or less necessary when shocking for algae? i.e. does it indicate the algae has been completely killed? I haven't paid any special attention to this but looking at my notes it has always been less than 0.5 when I stopped shocking (usually around 0.35ppm) - don't know if this is luck or coincidence!
 
#1, test your number if they are from a reliable test... Taylor drop reagents
#2, ignore the store numbers, your's are correct.

#3, continue shocking the pool by keeping your FC elevated to a shock level, 20FC based on a CYA of 50 until our test show you hold FC overnight (dark) and your CC is 0.5 or less (aka OCLT).

Anytime you allow your FC to drop below your CYA level minimum, you risk (encourage?) algae. So, with your 50CYA, your FC must ALWAYS be above 4PPM. When your FC is 0-3.5, you are not preventing algae.

This is direct contrast to what most pool stores and pool products recommend. The difference is, the pool products (in general) have a written disclaimer that the pools stabilization must be in the 30-50 PPM while the employee may not mention (or know) of this requirement.

http://www.hthpools.com/Docs/HTH_Pool_Care_Guide.pdf Page 7
 
That's why I said "seems". :)

If CC is still over .5, you are still not finished with the shock process, and is part of how you know when you are done shocking for algae. That's a good coincidence then that you were under! :)

As Jason was saying, 4 is your lower FC ppm number. So you want to stay above that, which means you have to start at a number over 4, as it will fall during the day as it is consumed by organics and the sun. If it requires going all the way up to 8 to stay over 4 all day, that's something you will have to test.
 
I just read this article. http://www.ppoa.org/pdfs/PrP_Cyanurics - Benefactor or Bomb.pdf

The issue is that if the FC is 2PPM, with a higher CYA, the ability of the chlorine to effectively kill the algae and other organics AND OXIDATE the waste is greatly diminished and could reach the level that it no longer control the algae and other bacteria in the pool (and your pool stays cloudy no mater what you do). Read the article and the concept of ORP. It has helped me to better understand the role of CYA in my pool.

I was considering adding more to my pool (I am currently at about 35 PPM CYA) but i think I will live with a little more FC solar loss for a lot more oxidation activity.
 
techguy said:
I just read this article. http://www.ppoa.org/pdfs/PrP_Cyanurics - Benefactor or Bomb.pdf

The issue is that if the FC is 2PPM, with a higher CYA, the ability of the chlorine to effectively kill the algae and other organics AND OXIDATE the waste is greatly diminished and could reach the level that it no longer control the algae and other bacteria in the pool (and your pool stays cloudy no mater what you do). Read the article and the concept of ORP. It has helped me to better understand the role of CYA in my pool.

I was considering adding more to my pool (I am currently at about 35 PPM CYA) but i think I will live with a little more FC solar loss for a lot more oxidation activity.
Click to expand...

Excellent article! When he says "Establish levels of 5 to 12 ppm CYA, or a little more, for your outdoor automated pool. Allow 20 ppm or so to provide good retention in a manually treated pool.", does that really mean dropping CYA below 20ppm? Also, the graphs show diminishing returns with CYA above 25ppm. If this is right (and I understand it) then it sounds like at 50ppm CYA it is not surprising to find an algae problem as effectiveness is so low (and useless at under 3ppm FC as in my case).

By the way, why do you think store tests are a poor guide - I haven't found significant difference - sometimes they are under my readings sometimes over but never by more than 10%? Part of that I put down to the time it takes to get the sample to the store. The kit they use looks a lot more sophisticated.
 
We have seen drastic differences in pool store tests, especially the CYA test. Some have even gone in with the same sample to the same store, husband once, then wife, and gotten different results.

The really sophisticated one uses lasers. We had that one a week or so ago. :)
 
RobbieH said:
That's why I said "seems". :)

If CC is still over .5, you are still not finished with the shock process, and is part of how you know when you are done shocking for algae. That's a good coincidence then that you were under! :)

As Jason was saying, 4 is your lower FC ppm number. So you want to stay above that, which means you have to start at a number over 4, as it will fall during the day as it is consumed by organics and the sun. If it requires going all the way up to 8 to stay over 4 all day, that's something you will have to test.
Click to expand...

Fair enough - it was useful to go over the shock process again. I'm still not sure of the connection between algae and CC. Does CC initially increase due to oxidising/disinfecting the algae? Is there a good scientific explanation of that process (and also how increasing FC causes CC to drop)?

I understand getting CC below 0.5ppm is good for health reasons (chloramines, although disinfectants, are dangerous and may be absorbed into skin) - just not sure I understand where it comes in the algae destruction process.
 
i guess it depends on the testing computer but I have not heard of any computer results anyone here seems to think are reliable.

I did notice the article two comments. One was they only found ORP testing to be a reliable electronic measure of the safety of the water and that using melamine CYA reagent was one option for causing CYA to precipitate out of the pool.
 

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