Preventing algae and when safe to swim

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Kendall.Nash

Active member
Jul 19, 2024
29
Little Rock AR
Pool Size
18000
Surface
Vinyl
Chlorine
Liquid Chlorine
Sorry in advance for this being long!
Thursday evening when I got my test kit, my FC was 7. Friday evening it dropped to 4. I added enough chlorine to raise it to 9. Tonight I just checked it and it was 5. Pool has looked very clean and clear but as I was about to add chlorine I noticed some algae along the edge of the pool and in a couple of corners. So I brushed the pool and added enough chlorine in to raise it to slam level (will retest in a couple hours) should be around 18. My CC has been between 0.5 and 0 each time I have checked it. PH has stayed at 75 and CYA has stayed at 40. I will post results after testing again tonight and in the morning to check for overnight chlorine loss. I have two questions though: 1. Pool math says my recommended FC level is 3-7 so I am a little surprised that I would find algae in the pool today. Do I just need to maintain a higher level than what I have the past few days? 2. My kids are really wanting to swim, at what point is it safe for them to swim after slamming it? How low does the FC need to get before they swim in it again?

Don’t know if it helps but the water temp has ranged from 81f-83f. Also I worked last weekend so I don’t remember if I brushed the side of the pool as normal, could missing brushing one week be what caused the algae on the edges and corners? I do have a dolphin robot vacuum that runs daily but it probably struggles to get in areas I found the algae. Lastly my water level was getting a little low so I added about 1.5 of water from the hose last night.
 
Kendall.Nash said:
1. Pool math says my recommended FC level is 3-7 so I am a little surprised that I would find algae in the pool today. Do I just need to maintain a higher level than what I have the past few days?
Click to expand...
In a word, yes. Your CYA is 40, your target range is 5-7. Test. Add enough chlorine, so tomorrow, WHEN YOU TEST, you are in range. If you are losing 3fc per day, then add enough to get to 6 (middle of range) + 3ppm loss + 1 for safety = 10. Then when you lose 3, you are still in range.
Kendall.Nash said:
2. My kids are really wanting to swim, at what point is it safe for them to swim after slamming it?
Click to expand...
If your FC > min and <= Slam level FC, it is safe to swim.
Kendall.Nash said:
How low does the FC need to get before they swim in it again?
Click to expand...
Slam level or below.

Brush regularly.
 
It's all about doing the SLAM Process properly. You said you have algae but your FC level is not being maintained at the proper level (16) until you pass all 3 SLAM criteria. That is key. Reference the chart below (SLAM portion) for best success.

full
 
I hadn’t been doing the slam process because up until tonight I hadn’t seen any algae so I was focused on maintaining recommended levels. I started the slam process tonight once I saw the algae, my question was more around why I got the algae in the first place since I thought I was maintaining correct levels.
 
PoolStored said:
In a word, yes. Your CYA is 40, your target range is 5-7. Test. Add enough chlorine, so tomorrow, WHEN YOU TEST, you are in range. If you are losing 3fc per day, then add enough to get to 6 (middle of range) + 3ppm loss + 1 for safety = 10. Then when you lose 3, you are still in range.

If your FC > min and <= Slam level FC, it is safe to swim.

Slam level or below.

Brush regularly.
Click to expand...
Kendall.Nash said:
I hadn’t been doing the slam process because up until tonight I hadn’t seen any algae so I was focused on maintaining recommended levels. I started the slam process tonight once I saw the algae, my question was more around why I got the algae in the first place since I thought I was maintaining correct levels.
Click to expand...
PoolStored said:
In a word, yes. Your CYA is 40, your target range is 5-7. Test. Add enough chlorine, so tomorrow, WHEN YOU TEST, you are in range. If you are losing 3fc per day, then add enough to get to 6 (middle of range) + 3ppm loss + 1 for safety = 10. Then when you lose 3, you are still in range.

If your FC > min and <= Slam level FC, it is safe to swim.

Slam level or below.

Brush regularly.
Click to expand...
Thank you, I will make sure I keep it closer to the 7 range than the 5 going forward, hopefully that corrects the problem.
I did retest after slamming it, might have over did it a bit, FC is at 21. So if I am understanding you correctly once I am done slamming it, the chlorine level just needs to get down under 16 for it to be safe to swim in again.
 
Yes I did, I used CalHypo instead of liquid chlorine because I have some left over from before that I am trying to use up and just keeping track of my calcium.
Pool math said adding 4lbs would increase about 13ppm. After retesting, it actually increased it 16ppm. But after closer inspection the Calhypo selected in pool math was 53% but the bags I actually have are 56% so that could account for the extra ppm. I also don’t know for sure how precise my pool volume is. I estimated it at 20,000 gal but it might be closer to 18,000.
 
Let's get pool volume correct.

53% vs. 56% is minimal. 4 lbs of 53% will raise FC by 16ppm in a 16,000 gallon pool. Adjust your pool volume to 17,000 now (accounts for the 56vs53 +/-). Then, when you are using a standard liquid chlorine product, add and test again after 30 minutes. If you overshoot, reduce pool volume by 500. If you undershoot, add 500 gallons. Retest again when you need to add more. When your addition test results match your expectations, you have a reasonable pool volume.
 
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FYI -don't know if it makes me a member of the #runhot club or not, but I've pretty much settled on keeping my FC at the top of the current recommended range, or even just a bit above. Currently my range is 4-9, but I run at 10. Easy for me with a SWCG - but more costly if you have to add Cl constantly. I do like the buffer, so if all the kids and their dogs show up unexpectedly or we have a sudden monsoon thunderstorm, etc. I will have more protection from an algae bloom. Spent too many decades in the bad old days fighting it.....
 
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I wasn’t able to test until about 6pm tonight, my FC had dropped to 13 and I saw a little more dead algae at the bottom so I brushed it and added 1 lb of cal-hypo, tested again a few minutes ago and it is back to 16.5 and 0 CC. Will try to test again in the morning before work to check for overnight loss. Checked pump, it’s still running steady so no need to backwash.
 
Tested this morning just before sun up,
FC 16 (0.5ppm loss)
CC 0

Tested a little after 5pm
FC 11
CC 0.5

Kids did go swimming this afternoon and it has been very hot.
No visible algae, dead or alive. Water is clear.
 
Would you recommend I continue to slam (raise chlorine level back to 16) raise it to 12 since I lost so much today, so that it won’t drop below 6-7 tomorrow before testing again, or leave it at 11, test again tomorrow and increase as needed?
 
I don’t know if I should start a new thread for this or just continue the conversation here but I have another question, this time about Alkalinity and PH.
I Tested everything today results are:
FC 8.5
CC 0
PH 7.3 (estimating because it looked slightly darker than 7.2 but definetely not 7.5)
Alkalinity 120
CYA 40
Calcium 150
I tested my PH about 6 days ago (I quit testing it every day because after about 4 straight test it had not changed, maybe not recommended) and it was 7.5 before.

My question is, should I be concerned about my alkalinity being high and my PH dropping a little or should I leave it alone since the PH is in acceptable range. My Alkalinity is high because before I came to this site and started testing myself, I listened to the pool store and put too much baking soda in it. Will it eventually drop on its own or at least as I backwash and refill or will that take too long? The process for lowering it seems complicated so I’m just trying to figure out if it’s worth it. If I’m not messing with the Alkalinity, should I use borax to increase PH or leave it where it is? Pool has been algae free, clean, clear and looks great! I just don’t want to overlook something that might cause me problems down the road or in the near future.
Thanks again for all the support!
 

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