Chronic mustard algae problem

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stephp25

Member
Jul 4, 2020
24
NY
We have been battling a problem with mustard algae in our pool since last summer. We closed the pool at the end of last summer and hoped that when we opened it this summer we would have a fresh start but that has not been the case. I bought the recommended testing kit and did the test today. We have some chemical balancing to do as our alkalinity and pH are too high and our calcium hardness is too low. We are going to adjust these over the next 24 hours. Our CYA is also lower than the 30 but not 0. The thing that I don’t understand is that we get a reading for combine chlorine of zero (taken both at night and in the morning) and an overnight loss of only 0.2 of free chlorine. We have tried different algicides and they all work for a few days but it always ends up coming back. We were planning to slam the pool but I’m not sure how long we do that for if our overnight pool readings are already showing that we meet the criteria for finishing the slam. If a slam is not needed, how on earth do we get rid of this algae once and for all? I am so confused because I was feeling like a slam with the way to go and was excited to get rid of this algae once and for all but our readings don’t seem to indicate that. Also, we have a vinyl liner pool so want to make sure that we won’t be bleaching our dark blue liner. Please help! Thank you!
 
SLAM Process
The first criteria is a crystal clear pool. If you have visible algae, you do not have that.
How have you done the OCLT if you received your test kit today? Do another OCLT. Raise the FC to at least 8-10 ppm for the test.
 
mknauss said:
SLAM Process
The first criteria is a crystal clear pool. If you have visible algae, you do not have that.
How have you done the OCLT if you received your test kit today? Do another OCLT. Raise the FC to at least 8-10 ppm for the test.
Click to expand...
I received the test kit a few days ago So I did the test last night after dark and first thing this morning before the pool had any sun. I didn’t realize that the test was supposed to be done with the chlorine so high. I will try that after we get the other chemicals balanced and will post the results. I am so desperate to get rid of this algae so that I can just enjoy the pool again. Thanks for your help.
 
I am completely confused and need some help. I am new to testing CYA level. When I test ours it is less than 30. When it is filled to the top, I can just barely see the black dot If I look really hard. My husband went the other day to pick up the chemicals to balance the pool and I guess just by habit he brought the water to be tested. They tested the CYA to be 40. I know from reading on this site that can be inaccurate. We decided to raise our chlorine level to 12 (using a CYA of 30 on the chart since we thought that would be safer to overestimate than underestimate). I guess not surprisingly we had difficulty keeping the chlorine level up yesterday and was having to add every time I tested (approximately every hour and a half). Now I am second-guessing the fact that I did not raise the CYA level before we started. Is it OK to add stabilizer to increase it now that we have already started the slam? I am not sure if the loss that we are experiencing is typical or more extreme because the CYA is low. Our pool is 20,000 gallons and we put about 1 2/3 gallons (12.5%) in to get the level up initially and went through an additional gallon during the day yesterday. Each time I tested, the level was below 12 even though I had added chlorine an hour and a half earlier. Typically it tested between 10 and 11. Also, while I understand that an initial big amount of chlorine needs to be added at the same time which is one reason to not raise the level with a chlorinator, can the chlorinator be left on during the da? I think I would still need to add liquid chlorine but I would think it would keep it a little more stable. I know that the concern is raising the CYA level to high that way but since mine is already low with that be OK?

Also, our OCLT was 1.5 last night and CC is .5. Should we still be seeing algae growth even when the SLAM is in process? We are seeing less than we saw before but there’s definitely still spots that appear in the pool. The water is clear but there are sporadic spots of algae on the Pool bottom which has been our problem all along since the first time we had the outbreak when the water was cloudy and the algae was growing on the walls and bottom of the pool before we knew what it was.

Sorry for the long post and thank you for your help!
 
Trust your testing. Pool store testing is notoriously incorrect.
Do not use trichlor (chlorinator). You will need to wean yourself off that. Only to be used when you are going to be away from the pool for several days.
The loss of FC you are seeing is very normal during the initial stages of the SLAM Process. The chlorine is being used up consuming algae.
If you can see the dot with the CYA vial full, then I would suggest you add 10 ppm more CYA. That should get you to 30 ppm total.
 
Thank you. So I will add CYA to Increase it to 30. I will have to be away from the house for 4-5 hours tomorrow and assume that the chlorine level will fall a lot as it is supposed to be sunny. Do you recommend that I put extra liquid chlorine in before I leave to increase it so that it doesn’t fall below the recommended level during that time period? Is more than the recommended amount bad? I have a vinyl pool and am always concerned about bleaching it but I am not sure if that is a valid concern with the levels we are dealing with. Also, I had read somewhere that you should brush the pool after adding the liquid chlorine if you have a vinyl pool because chlorine is heavier than water and will therefore sink. Is that accurate? It’s a pain to brush it every time we add chlorine but obviously I don’t want to bleach the liner.

Thank you so much for your help. You are so much more helpful than anyone that has helped me in the past year that we have been dealing with this!
 
It is best to not raise the FC level above your SLAM level. If you have to be away for a bit, so be it. Just test and dose back to SLAM level FC when you return.
 
So, we finally seem to have reached the criteria for ending the first part of the slam. We were still seeing a small spot of algae growing until last night (same spot was there every day), so it’s still okay to move on even though the algae was there 12 hours ago, correct? We have to do the extra high level for mustard algae for 24 hours. Should we start that this morning or should we wait until night when we will have 12 consistent hours at the beginning of the super slam that we don’t need to worry about burnoff from the sun?
 

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Does not matter. When you can maintain the MA levels the best. Remember to maintain higher FC levels going forward for a couple weeks as described in the article.
 
www.troublefreepool.com

Mustard Algae

Today We are going to show you how you can get rid of mustard algae in your pool. By following the steps in this guide and with no expensive chemicals.
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Chlorine/CYA Chart

EDIT: You will find a simplified and standardized version of this chart at Pool School, Chlorine / CYA Chart. Chlorine/CYA Chart by Chemgeek FC = Free Chlorine in parts-per-million (ppm) CYA = Cyanuric Acid in parts-per-million (ppm) CYA ........... Min FC3 ..... Target FC ...... Yel/Mstrd...
www.troublefreepool.com www.troublefreepool.com
"Yel/Mstrd Min" is 15% of the CYA level"
While at MA SLAM levels, no one should get in the pool water.
 
Just so I am sure that I understand correctly....... if my CYA level is 30, I should maintain the pool at 4.5 for two weeks after we do the 24 hours of the high level slam for mustard algae? If so, is it okay for kids to be in the pool when it is 4.5ppm?
 
stephp25 said:
Just so I am sure that I understand correctly....... if my CYA level is 30, I should maintain the pool at 4.5 for two weeks after we do the 24 hours of the high level slam for mustard algae? If so, is it okay for kids to be in the pool when it is 4.5ppm?
Click to expand...
I would make 5 ppm FC your minimum for a couple weeks. So you will need to raise the FC to 9 ppm or so each evening to achieve that.

It is safe to swim in the pool as long as the FC is at or below SLAM level FC (12 ppm for 30 ppm CYA) and above the minimum FC.
 
mknauss said:
I would make 5 ppm FC your minimum for a couple weeks. So you will need to raise the FC to 9 ppm or so each evening to achieve that.

It is safe to swim in the pool as long as the FC is at or below SLAM level FC (12 ppm for 30 ppm CYA) and above the minimum FC.
Click to expand...
Thanks for the clarification. So after the two weeks, we can go back to maintaining it at the minimum chlorine level of 2.5 (rounding up from 2.2)?
 
Remember, riding the minimum FC level begs for algae to reappear if you ever fall below it. So do what you say at your own risk. Many here use the Target Range in the FC/CYA Levels as their operating minimum FC.
 

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