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Algae On Waterline Boulders


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I cannot seem to keep algae from growing on my waterline boulders. FC level is fine and the rest of the pool is clear. Should I consider using an algaecide or should I just shock the pool? It is a saltwater pool.

Assume this is an in-ground plaster or pebble pool. How many gallons? How long is the pump/filter operated each day? Is the pool cloudy or clear? Finally, could you post the most recent test results?

pH

Free Chlorine (FC)

Combine Chlorine (CC)

Total Chlorine (TC)

Total Alkalinity (TA)

Calcium Hardness (CH)

Cyanuric Acid (CYA or stabilizer)

Salt PPM

For resolving an algae problem, the most important factors are Free Chlorine and CYA in PPM. If your SWG and filter are working properly and you have been maintaining a Free Chlorine level of 5% or more of CYA, there's less opportunity for an outbreak. E.g. If your CYA is 50 PPM, you'd want a minimum Free Chlorine level of 2.5 PPM.

Another thing that might help is regularly brushing the boulders in the area where they are getting algae along with your pool.

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It is a plaster pool ~11,000 gallons. I have been running the pump for 6 hrs/day, but increased it to 8 hrs today. The FC was fine yesterday, but when I tested a few minutes ago it came back as 0.

The water is clear

pH 7.8

Free Chlorine (FC) 0

Combine Chlorine (CC) 0

Total Chlorine (TC)

Total Alkalinity (TA) 80

Calcium Hardness (CH) 300

Cyanuric Acid (CYA or stabilizer) 40

Salt PPM 3300

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It is a plaster pool ~11,000 gallons. I have been running the pump for 6 hrs/day, but increased it to 8 hrs today. The FC was fine yesterday, but when I tested a few minutes ago it came back as 0.

The water is clear

pH 7.8

Free Chlorine (FC) 0

Combine Chlorine (CC) 0

Total Chlorine (TC)

Total Alkalinity (TA) 80

Calcium Hardness (CH) 300

Cyanuric Acid (CYA or stabilizer) 40

Salt PPM 3300

Zero chlorine? Though you may have only noticed algae on the boulders, it seems that you have the beginning of an algae bloom in the pool. I would recommend that you adjust your pH from 7.8 down to 7.0-7.4 with muriatic acid and commence shocking for algae with liquid chlorine or 6% bleach. Use one of the following tables to determine the right amount of chlorine to shock with (it's based on CYA/stabilizer):

Best Guess CYA Chart

Chlorine CYA Chart

For your pool (CYA 40 PPM) a shock level of about 15 or 16 PPM should work. A bit more will just speed things up.

The best way to successfully shock is to quickly add the full amount of chlorine needed and then HOLD it at that level while pumping/filtering 24/7 until the algae is gone and the water is crystal clear. If you're able to test Free Chlorine (FC) and add more chlorine to restore it to shock level several times a day, it shouldn't take long to get rid of it. You know that you're finished when your early morning FC is no more than 1 PPM less than nighttime FC. During this time, keep an eye on your filter and backwash (sand or DE) or clean (cartridge) when the pressure increases 10 PSI over normal. Keep skimmer/pump baskets cleaned out and brush pool walls (and boulders!) daily or more often. More is explained here.

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I would recommend that you adjust your pH from 7.8 down to 7.0-7.4 with muriatic acid

I have to disagree with the advice to lower the pH. I think that it is important to keep the pH above 7.5 at all time in a plaster pool. I generally target 7.7 to 8.0 and get good results. A higher pH will also protect metals from corrosion.

Also, I recommend the you increase filtration time and be more consistent with keeping the chlorine up

I have found that low pH and/or low CSI increases the risk of some types of algae, especially mustard algae. Low pH and or low CSI can also cause cloudiness due to plaster dissolving.

With a salt cell, you also have to make sure that the CSI does not get too high as it could lead to scaling.

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I would recommend that you adjust your pH from 7.8 down to 7.0-7.4 with muriatic acid

I have to disagree with the advice to lower the pH. I think that it is important to keep the pH above 7.5 at all time in a plaster pool. I generally target 7.7 to 8.0 and get good results. A higher pH will also protect metals from corrosion.

Also, I recommend the you increase filtration time and be more consistent with keeping the chlorine up

I have found that low pH and/or low CSI increases the risk of some types of algae, especially mustard algae. Low pH and or low CSI can also cause cloudiness due to plaster dissolving.

With a salt cell, you also have to make sure that the CSI does not get too high as it could lead to scaling.

doesn't chlorine work better at lower pH levels?

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polyvue strikes again! I tell you guys this guy is sharp. Never heard of the 5% fc to cya! Great stuff, I'll add that to my list!!! For the first time, a high cya makes a lot of sense!

Thanks for the kudos but I'm just an actor... I didn't write the play.

From the time I started researching pools in 2007 I read everything I could (my build was May-August 2007) from every source on the net. I learned the most from Pool Solutions / Pool Forum but found posts from waterbear, chem geek and others in this forum and TFP that answered questions I had about pool chemistry - questions that in some cases were not even being asked in forums that were more tightly coupled with pool products and services. That's why my loyalty, such that it is, goes to independent web sites.

My recommendation to target a FC level of ~ 5% of CYA is just a rule of thumb. is derived from this Chlorine/CYA Chart, and applies to pools chlorinated with SWGs and other automated devices.

I don't consider any of this gospel or dogma. If it doesn't work, question it...

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In this post I show graphs of the traditional hypochlorous acid and hypochlorite ion vs. pH and compare this against the true graph when Cyanuric Acid is present. When there is no CYA in the water, the drop in active chlorine (hypochlorous acid) levels at higher pH is substantial in percentage terms, but it's a moot point since in absolute terms this level is much higher than found in pools with CYA and is still more than enough for sanitation and oxidation.

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doesn't chlorine work better at lower pH levels?

When there is cyanuric acid in the water, the effect of pH becomes almost negligible.

Even when there is no cyanuric acid, the hypochlorite ions become hypochlorous acid as the hypochlorous acid is used up, because they maintain a constant ratio.

\ah ok. so all these products sold by the pool store stating first to adjust pH to 7.2-7.4 is not really neccessary when we have CYA in the water. especially 60-80ppm for like SWG's. B)

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There are reasons to have the pH at least in the 7.2 to 7.8 range, but it's not so much for sanitation. The average pH of tears is around 7.5 so having the water be closer to that in pH has less risk of stinging or irritation though most of that comes from either combined chlorines or from the low salt levels in the pool (osmotic pressure in the eye). A higher pH, especially above 7.8 or 8.0, significantly increases the risk of staining from metals in the water (or of scaling if other parameters such as CH and TA are high). A low pH, especially below 7.0, increases the risk of metal corrosion (or of dissolving plaster if other parameters such as CH and TA are low). Also, a lower pH will outgas carbon dioxide faster so pH tends to "settle in" higher than 7.2 for most pools and in SWG pools it can be easier to keep the pH at 7.7 then try and get below 7.5.

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