Green Algae and High Phosphates

by Julie
(Haven, MI)

Chemical readings

Free Chlorine 3.0
CYA 30
Phosphates 1000
PH 7.6
Alka Pool Store said OK, test kit said around 170
Acid - OK

Last Friday pool (25,000 gallong with sand filter) was clear, Sunday morning it looked like a green swamp. Pool store sold us Banish which we put in and added 1 gallon chlorine that evening. Next day no change. Shocked pool with 4 gallons liquid chlorine. Next day, no change.

Pool store said phosphates were high, so treated with Phosphate Remover (12 oz.) plus 4 oz. of clarifier. Still no change.

Have been backwashing throughout when pressure reaches 10 lbs. or more over normal. Vacuumed to waste on Wednesday and then shocked again with 4 gallons of liquid Wednesday evening. Thursday a.m. pool had changed to aqua color but water still very cloudy and some green areas on pool bottom...which we are starting to see again in shallow end.

Phosphates still 1000, so pool store said add 16 oz. more of phospate remover. Backwashed this a.m. and white stuff was in the water. Have also been adding a D.E. concentrate powder to the filter. Now are waiting another 24 hours to see what happens as Phospates are still at 1000.

Should we be doing anything differently?

Thanks for the question Julie

Your situation is the reason I put this site up in 2008. First, I'd like you to read this post which is very similar to your problem. It's long but worth the read.:

High Phosphates Over 1000 & Cloudy Water

Now for your issue. First thing is to stop using phosphate remover or clarifier. They're going to cloud up the pool, don't contribute to clearing it up, and don't tell you why the pool was cloudy or green in the first place.

According to Bioguard MSDS sheet, Banish is nothing more than a generic copper/metallic algaecide. It does nothing to remove/kill algae from the pool. If you're going to use an algaecide, it's best to use a PolyQuat 60. This contains no metal and is safe for vinyl liners. Keep this in mind because I'll return to it shortly.

First is to get the total alkalinity (TA) down to 80 - 100ppm, 120ppm being the top. It's 0.8 qrts. of acid per 10,000 gallons of water to reduce the TA 10ppm. You have a 25,000 gallon pool and the TA is 170ppm. You need to reduce it at least 70ppm so you'll need 3.5 gallons of acid. Add 1.5 gallons of acid to a bucket filled with pool water and stir with a stick. With the pump OFF add to the deepest part of the pool and very gently sweep the bottom to break up any hot spots of acid. Allow to sit for 3 - 4 hours, then pump back ON to FILTER for 8 - 10 hours and retest. Make another adjustment if needed. Test the pH as well and make sure it doesn't go below 7.0.

Pool Alkalinity

Total Alkalinity

The CYA is fine so don't touch that.

Now it's time to shock the pool. Shocking is a process, not an event. The trick is to get AND keep the chlorine above 10 - 12ppm for a period of time. You'll need to manually dose the pool with chlorine to keep it at 10 - 12ppm. Make the adjustment at night, then retest in the morning. Be sure to have the pump running 24/7 and backwash once per day. You can go up to 15ppm, but no higher. Anything after that and you're wasting money and chemicals.

You'll know this is working because the pool will go from green to a white/gray cloudy and the chlorine will begin to hold better. First you may lose most, then 2/3, then 1/2, and so on. Once you only lose 1 - 2ppm of chlorine 8 - 10 hours after the last application you know the algae is dead. Now it's just a matter of filtering and backwashing once per day.

Green Pools

Swimming Pool Chlorine

Pool Shock

Pool Chlorine

Back to the algaecide. You can use a PolyQuat 60 during the shocking process as insurance against another algae bloom. Remember, an algaecide isn't to kill already existing algae.

Here's a good post about putting DE in a sand filter:

Is Using DE In My Sand Filter Good?

To sum it up:

TA to 80 - 100ppm
pH to 7.2
CYA is fine
Shock the pool

If you would like personal assistance, I do phone consultations for a donation of your choice. It makes things go much faster. If you choose to not go that route, we can correspond by email.

Contact Me


Hope this helps and have a great Summer.


Comments for Green Algae and High Phosphates

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Jul 09, 2012
Progress With A Green Pool & Phosphates
by: Julie

Thank you so much for your advice. We have made a great deal of progress by shocking again Friday night with 4 gallons of liquid after we disassembled the filter and removed all the phosphates from the top of the sand and backwashed.

We also added 32 oz. of PolyQuat 60 when we shocked. Saturday morning pool was finally a bluish color although still very cloudy, but we could now see the bottom in the shallow end. Continued to backwash and have kept 2-3 tablets in the skimmer at all times. Added more Ecko cleaner to filter and continued to backwash every time pressure exceeded 8-10 lbs. over normal. Added another gallon of liquid chlorine Saturday night and this morning the drain in the deep end was finally visible!

We vacuumed to waste and continued to run the pump 24/7. Pool store said FC at 10, Alka at 107, pH at 7.5 and CYA at 38. Bought a Taylor 2600 test kit and it says FC at 20, Alka 120, pH 7.6 and CYA at 30. Could the Taylor kit be off that much on the FC? Phosphates are still over 1000 according to the pool store test.

From everything we have read on your website, which is awesome by the way, we are striving to keep the FC and CYA in balance and add PolyQuat 60 weekly to offset the phosphates.

Thank you again for your input and your fantastic website.


Jul 09, 2012
Green Pool & High Phosphate Update
by: Robert

First, it's good that you're shocking, keeping it shocked, and testing. Be sure to keep a log as to times, amounts, etc... This way you can see a pattern develop. PolyQuat 60 is a good algaecide, but when you get your pool perfect again, you shouldn't need it.

You're backwashing and vacuuming to WASTE which is good, but remember that you lose about 200 gallons of water each time you backwash and when you vacuum to WASTE that water is leaving the pool. Remember these two things: don't allow the water to get below the skimmer or the system will draw in air. And 2nd, you're going to be topping off the pool with fresh fill water which may dilute the chemicals and/or skew the readings.

Many pool stores use the K-2006 kit to test water. It does take some time getting used to it, especially the swirling technique. Many people kinda shake it or slosh it around, but it takes a good swirl to get the right numbers. I've used the kit since 1999 and have trained about 30 lifeguards at the Y since 2008 with the same kit. It takes them about 2 weeks before they really get the hang of it, and that's doing 5 - 10 tests per day. I've done literally thousands of tests.

If it's your first time using it, I'd probably go with the pool store readings, or maybe get a 2nd opinion from another store, if available.

You wrote "Pool store said FC at 10, Alka at 107, pH at 7.5 and CYA at 38." These are very good numbers. What you're doing is right, just keep the chlorine above 10ppm and backwash once per day or when the psi gets 8 - 10 lbs. over the just backwashed pressure.

About the phosphates, if you haven't read the post below about them, please do. Too many pool owners get sold on all of these additives and extra chemicals that they don't need. I've had questions from people that have spent upwards of $1000 in a month trying to clear up their pools.

Now about the tabs in the skimmer. It's alright to leave them in the skimmer during this process, but get a tab floater after the pool is good to go. Once the filter stops the tabs keep diluting and you get a nice pocket of acidic water. When the pump turns back on that acid water goes directly into the system. Over time it can dissolve gaskets and o-rings which can cause leaks. Also, the tabs are probably Trichlor tabs which have CYA in them so keep an eye on the stabilizer level. The range is 30 - 50ppm CYA. If it gets too high, over 70 - 80ppm, the only thing you can do is a partial drain and refill. There's no chemical to reduce CYA. A tab floater is about $15.

The PolyQuat 60 won't offset the phosphates, it's just a nice little insurance policy against another algae bloom. But, when you have the chemicals right in line, there's no reason to use an algaecide. The exceptions are if you go out of town or are unable to check the pool twice per week and make manual adjustments. I get this question from people who travel alot.


Jul 10, 2012
Clearing Up Our Pool
by: Julie

Thank you again for all your great advice. I have read almost everything on your website and it has been such a great help. I have looked everywhere, however, and haven't found anything about "safe" swimming levels of chlorine.

We have grandchildren 3 to 7 and other children 10 to 17. Is a chlorine level of 10 safe for them to swim? Also, we have had a party planned for this Saturday, so I would like to let the chlorine levels start to drift back down toward the end of the week if it is not safe to swim in at the "10" level.

I have Googled this question and have not found answers that range from "yes" it is safe, to "no way"! Please help!



Jul 10, 2012
Update For Green Pool
by: Robert

Concerning the chlorine level, I advise pool owners that a level of 5.0ppm or below is safe to swim in.

I kept the Y pool at 3.5 - 4.5ppm and people never complained about burning or itchy eyes or noses. If your pool level is 10 right now, it should be fine by Saturday baring any unforeseen events.

Jul 10, 2012
Update On Green Pool & Phosphates
by: Julie

Readings this morning were:

Chlorine: 12 (it was still 8.5 last night so we added 1 gallon)
CYA: 44
pH: 7.5
Alka: 130
Hardness: 210
Phosphates: Approaching 2500

Water is clear and very little algae on the bottom this morning. I had our well water tested and it is fine except it has a high level of alkalinity. Considering I filled the pool this morning after taking the water sample for the above readings, should we address the alkalinity now, or wait until after the party?

Also, I know you are not a big fan of removing the phosphates, but we live next to a golf course and since the well water has a very low level of phosphates, we are assuming they are coming the from golf course fertization and/or the crop dusting.

Your advice has been so very helpful...we just can't thank you enough!


Jul 10, 2012
by: Robert

Yes the TA is just a bit high at 130ppm, but nothing to worry about. If the fill water TA is high you'll need to stay on top of keeping the pool TA down. Due to evaporation and splash-out the pool will be filled with high TA fill water so it stands to reason that the pool TA will slowly creep up. A couple extra gallons of acid to have on hand would do you well.

Remember it's 0.8 qrts. of acid to reduce the TA 10ppm per 10,000 gallons. You need the TA to decrease by another 30ppm which is very easy. One or two more shots of acid should do it. I'd do 2 qrts. to start with in a circular motion in one spot with the pump OFF, gently sweep, sit for 3 - 4 hours, then pump on FILTER and retest in 8 - 10 hours.

Also remember that when you only lose 1 - 2ppm of chlorine after 8 - 10 hours of FILTERING and after the last dose of chlorine, the algae is pretty much history. Now just FILTER 24/7, sweep, and backwash.

When I would shock the Y pool I'd need to jam the chlorine down from 15ppm to 8ppm with Thiosulfate which is a chlorine neutralizer. I'm not a big fan of doing this because it can skew the other readings and is fairly expensive but I didn't have a choice. I couldn't wait 5 - 6 days for the chlorine to come down. People had to use the pool. The other two ways for the chlorine to come down is either naturally through heat and sun or partial drain and refill.

Phosphates are non-poisonous and safe to swim in. To the best of my knowledge, in pools, they pose no health risks. Excessive phosphate consumption is linked to cardiovascular diseases in the general population but this is because of the fast food industry and soda like Coke and Pepsi. You're not swimming in McDonald's and Pepsi so I think it'll be fine.

There are many articles on the internet about the dangers of "high" phosphate levels in pools. While it's true that algae thrives off phosphates, if you kill the algae with chlorine and keep the water balanced, and phosphates are harmless to swim in, then why do I need to spend $1 an oz. on another chemical?

Remember this post:

High Phosphates Over 1000 & Cloudy Water

Anyway, I'm glad you were able to get your pool back and have fun this Saturday.


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