Persistent surface algae - came back after SLAM

[jeffbrig]

~15k plaster pool in S Florida. I have a yellow/green algae that likes to grow on the shaded walls on the south side of the pool. Here are my general chemistry numbers:

FC 5-7 daily (I chlorinate with 10% liquid)
CYA 40
pH 7.8
TA 70
CH 250
Temp 85

My pH tends to the high side, so I tend to keep my TA towards the low end, and let pH get as high as 7.8 before adding acid.

Last year when I had this surface algae, I repeatedly would raise FC to SLAM, watch everything clear, lower to regular, and watch it reappear. I finally got frustrated and threw in a bromine treatment (not by TFP recommendation ), which nuked the algae, but the pool was terrible holding chlorine after that. I understand that bromine converts to bromide in UV, and chlorine is used up by the "recharging" of the bromine. So not a good solution for an outdoor pool in full sun. But it did get rid of the algae.

Fast foward a year, and the bromine appears to have finally worked its way out of the pool. The algae started creeping back in. It's only on the shaded walls. Brushing kicks up a small cloud, but some color remains on the plaster and does not come off with brushing. The water is always clear, it's only been a surface problem. And only on the shaded portions of the pool.

On 4/30 I started another SLAM. Based on CYA set at 40, I raised FC to 16, and kept it there for three days. I tested the water the 4-5x a day, and made sure to stay above 16. Brushed like mad each time too. OCLT didn't pass (just barely) on the 2nd night, so I went another day just to be sure. Then, to be ABSOLUTELY sure, I raised FC to 24 (Mustard Shock Level) for the next 24 hours on 5/2-5/3, before letting it return to normal levels.

By 5/7 I was back to daily dosing chlorine. The minimum FC level for a CYA of 40 is 3, and I never went that low.

I get up this morning and go out to clean the pool, and lo and behold, I see the algae starting again in the shaded portions. My water is, and has been, crystal clear. With the pump off, you could read a newspaper placed on the bottom of the deep end.

I don't get it. I'm doing everything by the book. Everything. I never let my chlorine drop below the minimum level for my FC. I did the SLAM to the letter, spotless pool - passed the OCLT, did 24hrs at Mustard Algae level, and yet my algae problem came back in less than a week at normal FC levels.


What next? I'm sure I'm going to get the regular boilerplate - "you need to SLAM...again". That's not really what I'm looking to hear. I'm more interested in exploring what type of algae I'm dealing with, and why it seems resistant to normal FC levels and even a Mustard Algae SLAM level, and even shrugs off aggressive brushing. Because without finding something different to do to address this particular problem, I expect it's just going to come back....again.

 

[pabeader]

There are a couple of things to consider. Is that area of the pool getting good circulation? Do you brush that area often enough? It isn't necessarily how hard you brush, but maybe how often.

 

[jeffbrig]

It's the entire 30' length of the south wall of the pool. So the flow varies a bit from the area closest to the return, and the far end of the pool, and no difference in the algae based on that flow. But in general there is a good circular flow around the entire pool. Let me know what you're thinking....if you think it would be helpful, I can take a syringe and draw a water from against the wall and see if the chlorine test of that water differs significantly from the pool at large.

What do you consider often enough for brushing? Heck it was 3-4x a day during the slam, and that finished less than a week ago. So in less than a week, I have algae spots redeveloping.

 

[ka8yiu]

Could it be pollen collecting in/on the pool?

 

[Richard320]

It sounds like you have some particularly robust yellow algae spores in your pool.

Have you checked behind the light? Check out the pictures in Think I may have found my FC demand issue?

 

[jeffbrig]

Could it be pollen collecting in/on the pool?

Unless my pool's gravity somehow pulls pollen to the south wall, but not the bottom, I think we can rule out pollen.

- - - Updated - - -

It sounds like you have some particularly robust yellow algae spores in your pool.

Have you checked behind the light? Check out the pictures in Think I may have found my FC demand issue?

No, I haven't removed the light fixture. The last time I had it out (couple of years back) it was clean. Might be worth taking a look this weekend...

But let's say I do find something...does that explain why algae would return in other areas of the pool? I mean, we expect pollen spores to land in our pools all the time, right? Chlorine is supposed to prevent that algae from growth. I'm not having trouble holding FC at normal levels, it just don't seem to be stopping new algae growth.

 

[Richard320]

Mustard algae favors shade. Spores travel around the whole pool and settle where they find a comfy spot. That's why I say robust, because normal FC level isn't enough to kill it.

Also, if you do find some behind the lights and go to mustard shock level, toss any toys and tools in to soak. Cover, too, if you use one.

 

[pabeader]

Circulation - I was thinking maybe the area is in a 'dead zone'. Where the water is flowing around but not into. Doesn't sound like you have that.
Brushing - How many times a day, since the SLAM? Since the area is known to be troublesome, it may need more attention on a regular basis.

 

[duraleigh]

You will not like hearing this but there is a 99% chance you have mustard algae. Since it reappears, that means you are not brushing vacuuming and holding the FC consistently high enough to keep it away.

Your issue is not that complicated and your pool will be crystal clear if you dedicate yourself to brushing, vacuuming and keeping you chlorine adequate

 

[jeffbrig]

You will not like hearing this but there is a 99% chance you have mustard algae. Since it reappears, that means you are not brushing vacuuming and holding the FC consistently high enough to keep it away.
Your issue is not that complicated and your pool will be crystal clear if you dedicate yourself to brushing, vacuuming and keeping you chlorine adequate

I agree that it's likely mustard algae. Where I'm disheartened is that it seems the TFP SLAM, OCLT passed, then +24 hrs at Mustard Algae FC does not appear to be sufficient to kill it off. Sure, it disappeared while FC was elevated, but it came back in less than a week - and during that week FC never drifted below recommended minimum levels for my CYA. That tells me that even 4x daily brushing and 24 hrs at an FC of 24 (CYA is 40) was not enough to kill everything off. The whole concept of mustard algae shock level just seems less of a sure thing than a general SLAM. Has anyone on here successfully battled mustard algae and won using the procedure as stated? Or is this procedure a hypothesis we're still trying to prove/disprove?

I don't mind slamming the pool again if that's needed. I don't however, want to do the exact thing I did last time, and hope for a better result (that's the textbook definition of insanity). I followed the SLAM procedure to the letter. I then followed the 24hr mustard algae shock process to the letter. It wasn't enough. So I'm asking....what should I do/try differently next time?

If this type of discussion belongs more in the deep end, I'm happy to make another thread, or we can move this one there.

I'm also trying to locate my underwater camera, so I can take some decent photos to share...