New Algae Growth When I Don't Think It Should Be There

mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
We opened the pool in April. Got everything balanced very nicely. After brushing walls, would repeatedly see significant rapid algae growth every 3-4 days only around the wall light at the deep end, even when the water was still around 68F (VERY cold spring here in MD). Like 10 square feet of green/black algae growth on both sides and below the light. So, I went in the pool about a week ago (+78F) with a screwdriver to open the light up and sure enough, there was a lot of old nasty gross algae muck inside the circular bezel holding it in place and all over the rear of the bulb itself. Cleaned the entire structure with a tooth-brush. The skimmer returns and rope holders a clean. No other hiding places for algae found. When cleaning the light....Most of it went into the pool water. Most of it got skimmed out first, then what was left filtered as it's no longer int he pool. These are the last readings from the "pool store" and they are in close agreement with my TFT Test Kit readings from three days ago:

ph=7.7
CC= way less than 0.5, almost negligible
TA=75
CH=288
CYA=62
Copper/Iron/Nitrates=0
Phosphates: 815
Salt: 3110
Saturation Index = +0.3

Three days ago after TFT test kit measurement and thorough brush of the pool I Raised the FC up to about 9-10 with Cal-Hypo, set my IC-40 CL Gen unit to make CL 100% of the run time (which is 10 hr/day), added a little acid just to get the Ph down to 7.6. I also added 40 pounds of salt because my IC-40 says it needs to be at 3600 ppm salt concentration for optimal results.

After three days I have a small number of green algae by the light only....about 1 square foot. FC is around 7.5. Ph is around 7.8 (added some acid).

So, I'm trying to figure out: why did the algae reform in three days by the light? What am I missing?

I have not backwashed sand filter since I cleaned the algae scum from the light: could that be a reason?
Get it out of the pool system?

Pool water looks clear, maybe a little hazy but pretty darn good.
Thanks

Mark in SO. MD
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
43,003
Tucson, AZ
Marty hit the nail on the head. You have never fully eradicated the algae because you've never completed the slam process.
 

mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
Copy that. I will SLAM. I know how to and did it once before. I guess I'll SLAM every year upon opening even if the water is clear (which it was because I usually open early to use the Jacuzzi whilst it's still cold out)....thanks folks
 

mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
And I assume I can leave my IC-40 SWCG inline, just disconnect the power from the nearby power supply box. In other words, SLAMMing at CL= 24-26 PPM will not hurt the internal components of the SWCG. I have a bypass pipe with same dimension/unions that can be inserted instead of the SQCG.
 

mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
I'd like to continue the thread with results of the SLAM: Cleaned and brushed the pool and backwashed filter. Water slightly cloudy due to some algae pockets on the walls now in suspension. Pump on....Adjusted Ph down to below 7.5, looks like 7.2. Measured FC with TFT Test Kit and it was 6.5. Since I believe my pool is 23,000 gallons, I went to Pool Math and calculated with a CYA of 62 I needed 4 gallons of 10% liquid pool shock/bleach. I have some 73% Cal-Hypo too but wanted to use the liquid. With the sun on the horizon, I added 4 gal 10% liquid Cl2 very slowly, over a 15 minute period, distributed all around the pool. HERE IS MY SURPRISE: I measured FC about 1 hr later and it was 40.5 ppm. Wow, I expected 24. So, either my pool is not 23,000 gallons or the CL2 I bought from Wal-Mart more than 10%, see picture below. Any other possibilities?

The 23,000-gallon number comes from the documents supplied by the pool installer in 2005, and also when we decided to replace the tile and coping in 2017 the water trucks delivered 23,000 gallons to refill (we are on a deep well, not a good idea to take out 23K gal at one time). So it thinks it's accurate. I also drained the spa water to the pool by adjusting pool diverter valves...before adding Cl2...then refilled it from pool water to make sure both pool and spa were at the same concentration.

THE GOOD NEWS is that my FC is 36 this morning and that makes me happy. And the pool as one would expect looks GREAT...noticably more clear than yesterday. Which also makes me very happy. So, I'll continue to check the FC level during the day and watch it drop (My SWCG power line is disconnected)....and keep it above 24.

Does anyone have a clue about another possibility as to why I got 40.5 when pool math said I should get 24?

Thanks to all, MarkIMG_7768.JPG
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
20,592
Laughlin, NV
I assume you have been running your pump since before you added chlorine yesterday?

Pool volume is the most suspicious. Take a few measurements and use the calculator in PoolMath
 

tessv

Gold Supporter
Bronze Supporter
Jul 3, 2015
161
Auburn, KS
One thing I have done is to inadvertently record the total number of drops to clear while doing the FC test—forgetting to divide them in half 😊... could that be it??? Would result in numbers closer to what you predicted...
 

Frodo

Member
Jul 5, 2014
14
Central New Jersey
One thing I have done is to inadvertently record the total number of drops to clear while doing the FC test—forgetting to divide them in half 😊... could that be it??? Would result in numbers closer to what you predicted...
Could be it, but how would you get the 0.5 FC if you didn't divide. Test again and see.
 

mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
Tessv: I have been dividing by two. For sure. And counting very carefully.

I measured FC at 0900 and it was down from 36 at 0600 to 34.5....the sun does not hit my pool directly till AFTER that time so it has to be algae oxidation. I assume the TFT 100 test kit is accurate at such high levels of Cl2.

Mknauss and frodo: As to the pool volume, unless the H2O company lied to me when they said they put in 23,000 gallons after re-tile work....could be that. I'll go double check the pool mfg. docs too. In a file cabinet somewhere in the basement. I can always break out my tape measure too and estimate. It is a teardrop-shaped pool.....

I'm going to take a few measurements after the SLAM is done per suggestions here. I'll let it get down to 7 ppm and then calculate what it would take to get it to 14....and add that amount. Or some such range endpoint. Thanks all.... :) much appreciated.
 
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mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
Pump has been on "almost continuously" since yesterday when SLAM process was started. It inadvertently stopped about 3 AM this moring, I restarted it a 6 AM.
 

mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
UPDATE: First morning CL went from 40.5 (the night before) to 36 (at 6 AM next morning).....continued to monitor CL2 during the day yesterday and it never dropped below 30.5 at sundown. I measured it on the second night at 9 PM and it was 30.5. This morning it was 26. So, I have to continue the SLAM. I kicked up the CL level a bit...to around 30. Water is perfectly clear (I can see the Phillips screw heads at the drain clearly) and CC is less than 0.5.....I hope the SLAM ends tomorrow morning. :)
 

mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
Also, I brushed the pool 3x yesterday and at around 5 PM I noticed the Hayward sand filter pressure had gone from 18 (normal pressure after backwash) to 27!!! Wow, it usually takes 5-6 days to go from 18-22 under normal circumstances....this was higher than I had ever seen it except when opening the pool and it's filthy. I am assuming this is due to unseen microscopic particles of dead algae clogging it up (and I usually add a little DE after a backwash). The water is crystal clear....
 

mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
Well, it's 2:30 PM on the second SLAM day. I brushed the pool this AM. Took a CL reading at 2 PM and CL was down to 24. I added more (Cal Hypo 73%)...about 12 ounces by VOLUME, as I understand with a CYA of 62 I don't want to go below 24 until SLAM is completed. I noticed the pump pressure building a bit....looks like I'll need another backwash, just like yesterday...I hope all those little microscopic DEAD algae blooms are clogging the filter. Very rare I ever need to backwash the filter on back to back days. The pool looks perfect. CC is very low, negliable using TFT-100. Let's see what I get for my next OCLT.
 

mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
UPDATE: WHOOOO HOOO. :) Cl2 was 26.5 at 9 PM last night and it's 26 today at 0630.... measured twice just to make sure I counted correctly. I'm done. CC practically nil, and the pool never looked better. SWCG re-connected, filter back washed and everything clean. I expect now that algae bloom around the light at the deep end won't be seen again. Thanks to all!
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
43,003
Tucson, AZ
Great! Keep you FC high enough for your CYA and break occasionally and you should be good all summer.
 

mmcwhorter

Bronze Supporter
Jul 13, 2015
108
Huntingtown, MD
POST-SLAM UPDATE: I went away for 15 days to Europe on a biz trip. Had the neighboorhood kid clean the skimmers and scoop the leaves off the surface daily. Set my IC-40 to 100% before leaving. When I returned the water was perfectly clear. Could see the date on a quarter at the bottom of the pool. Backed the IC-40 down to 80% and it liked around FC = 13, then backed it down to 60 and it likes FC = 9......VERY HAPPY WITH THE SLAM RESULTS!!!!! THE POOL HAS NEVER LOOKED BETTER. FC=8, CC=NIL, PH TYPICALLY AROUND 7.6, TA=80, CH=325, CYA AROUND 40. Thanks.....

Now I get it....when we take the cover off in March.....SLAM THE POOL and it'll be good for the rest of the year. Duh!