Algae spots on plaster, confusing results

[Gecrabill]

Fc:3
Cc:1
Ph:7.5
Ta:70
Cya:60

Went out this morning to a skimmer full of leaves and two algae stains, one on the steps another in the middle of the pool at the bottom. tested chlorine, prior to the pump and swg starting this morning and FC was 3, cc:1. I have been keeping my fc around 7 since the weather turned cold (well, cold for florida, water temp 70). A few weeks ago I decided to lower my fc so I shut off the swg for a day. I turned swg back on at a low percentage to keep fc at 4. It seems like everytime I decide to drop my fc below 6, I get some type of algae problem...this is only the 3rd slam since the pool was built last April. Well we got a spell of hot weather the last 3 days and some wind yesterday which blew a ton of dead leaves in the pool. So I did the trek to cvs and got the 8.25% bleach in, enough to bring up FC to about 28 ( I tend to overshoot on the slam by a couple ppm) at 11:30am. Well after a good brushing, the stains were gone about an hour after I added the initial amount of bleach. I also had the swg on 100%. Only have added 120oz of the 8.25% since the start of the slam. Just tested my Fc and I am at 34. So I just turned off my swg.

Ok, there's the background... I guess my question is, why do I keep getting algae problems when I bring my fc to 4? I've read over pool school and do pool math consistently and 4 seems like it should be ok? Also is it normal for algae stains to go away so quick?

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Ahh forgot
CH:825... Yes quadruple dope checked... Fill water is high.

 

[chem geek]

What kind of algae is it? Is it green algae, yellow/mustard algae, or black algae? Green algae will be anywhere in the pool including in sunlight; yellow/mustard algae prefers shade and puffs up like a powder if you brush it. Black algae stays in one place attached to the plaster.

Is the algae showing up in areas of poor water circulation?

 

[Gecrabill]

I believe that it is green algae. When I brushed it, there was no dust or anything, it was a greenish yellow color. Both areas get full sun for 5 hours a day. Both are in areas with decent circulation. The one on the steps gets indirectly hit by a return right above it.

 

[duraleigh]

I guess my question is, why do I keep getting algae problems when I bring my fc to 4?

The application of TFP methods is certainly not absolutely precise. a 4 ppm is not a rock solid guarantee but rather a guideline that most always works.

Since your maintenance of your pool water and your methods indicate you understand the process, why not simply keep your FC @ 6 ppm. The increased cost is virtually insignificant when you consider how much it costs to SLAM the pool.

When I brushed it, there was no dust or anything,

What did it do? Did it disappear? Did it remain as if it was untouched?

 

[Swampwoman]

^the poster's CYA is 60.

For 60, the MIN is 5 ppm, the TARGET is 7. I suspect that is why, Gecrabill, you keep getting algae at 4 ppm.

See the CYA-chlorine chart in pool school. The reason I checked is because I keep my cya around 40 and don't let mine go below 4, so I knew something was amiss. Its all relative

 

[pwrstrk]

^the poster's CYA is 60.

For 60, the MIN is 5 ppm, the TARGET is 7. I suspect that is why, Gecrabill, you keep getting algae at 4 ppm.

See the CYA-chlorine chart in pool school. The reason I checked is because I keep my cya around 40 and don't let mine go below 4, so I knew something was amiss. Its all relative

OP has a SWG. That is where the target of 4 is coming from. The min FC for a SWG at 60 ppm of CYA is 3.
I agree OP should maintain the FC where it was at 6.

 

[Swampwoman]

^thanks for clearing me up on that. Is the difference just because the chlorine dosing is steady with a SWG? Because to my mind, I'm not sure what else the difference might be to cause a different cya ratio...except the salt. With higher ph drift too, I guess I'd think a higher value might be safer in terms if sanitizing efficacy, since the efficacy increases at lower ph...must be something at work here that I don't understand certainly not the first time

 

[chem geek]

Is the difference just because the chlorine dosing is steady with a SWG? Because to my mind, I'm not sure what else the difference might be to cause a different cya ratio...except the salt. With higher ph drift too, I guess I'd think a higher value might be safer in terms if sanitizing efficacy, since the efficacy increases at lower ph

It may also be the salt level since even pools with automatic dosing systems such as peristaltic pumps and The Liquidator need to use the manually dosed guidelines. Obviously this particular pool isn't following either since it is needing an FC/CYA ratio of around 10% to prevent green algae growth instead of the usual absolute minimum (at all times) of 5% for SWG or 7.5% for non-SWG. The loss in sanitizing efficiency at higher pH is only a 15% loss from pH 7.5 to 8.0 due to CYA being in the water that acts as a hypochlorous acid buffer.

If this problem with algae in this pool forming below 10% FC/CYA ratios only occurs when the water temperature is colder, then there is an effect where CYA binds the chlorine more strongly at colder temps (and likewise is bound less strongly at higher temps) and at 70ºF there may be only half the active chlorine compared to 80ºF (likewise at 90ºF there may be roughly double the active chlorine level). It could be that this pool is rich in algae nutrients (phosphates and nitrates) so that this temperature effect is being seen. The FC/CYA table guidelines are set based originally on Ben Powell's observations which likely were with pools mostly in the 80-88 temperature range so going below 80ºF with a pool that is "on the edge" due to high algae nutrients may require the higher chlorine level to prevent algae growth.

The pebble finish may also be more susceptible to algae compared to a smoother finish due to water circulation differences near the surface. Weekly brushing of the pool may help to dislodge any algae to expose it to the chlorinated water away from a surface pocket.

 

[Gecrabill]

Thank you all for the detailed responses. This helps alot. I know there is alot of food for algae here.. It is florida. I thought with the lower eater temp, I would need less fc, but I see now that it isnt true. I am not going on night 5 of slamming. I have lost 2ppm both last night and the night before. Hopefully tonight is the finally and I can begin dropping to normal range. We have been having ubseasonably warm weather here this week and into next. I will be keeping the FC in the 6 range from now on. Its very confusing when the swg says to maintain at 3ppm, the interior finish company says 3ppm, and at 1ppm above at 4, I consistently have problems.

 

[PAGirl]

I have seen that brushing makes a difference, as Chem Geek mentioned above.

 

[Gecrabill]

How much effect on the ofclt would small seeds in the pump basket have? Only reason I ask is that I am on day 6 and the past 3 nights have been getting a consistent 4 to 5ppm loss vs the 2ppm loss earlier in the slam. I even turbo fueled the FC level and brought it up to 35? Any thoughts? Problem is, if I try to empty out the pump basket, the seeds will just flow back down into the pipes once I open the air bleed in the filter?

 

[JasonLion]

A few seeds, even a handful shouldn't be a problem. But an entire skimmer basket full of them could possibly pose an issue, though only sometimes.

Raising the FC level makes the FC test a little more error prone, but otherwise isn't required, though it should help if it does anything.

When you go several days with things looking good except for losing a little FC overnight, it is good to start looking for hidden places and making sure you really brush everywhere and making sure you have vacuumed up all of the dead algae. Areas behind ladders can sometimes miss getting brushed, and light niches sometimes need to be opened up so you can brush them out.

 

[Divin Dave]

Yup. Hidden Places. algae behind Light Niche will definitely produce results like you are seeing. If you have any water features, the algae will grow in those pipes if they havent been running for a while. Sometimes its hard to find it all.

 

[Gecrabill]

Back to 5 ppm loss last night. Have run the water features and brushed 3 times. FC 43 last night. FC 38 this morning. Kind of curious, wouldnt algae need sunlight to grow? How does it get behind the niche? Also, how do yiu clean the niche?

 

[pwrstrk]

There should be a single screw at the top of the light fixture.

Turn the power off to the light by flipping the circuit breaker.
Remove the screw and take the light fixture out. There should be enough wire to set it on the pool deck. You may have to pry it gently lose from the niche with a flat blade screw driver.
You should be able to do this without getting in the pool. This is a good place to look for algae.
Why are you using 43 as your shock level ? Your CYA level is 60 and the shock level for that is 24. At that level your wasting chlorine IMO.