I have a salt water pool and I balance the pH once per week. Last Wednesday I lobbed in the usual quantity of pH-down powder and the next day I had a nasty green algae bloom. I checked the pH and it was about 7.1, i.e. usual after treatment.

I then gave it a chlorine shock, about 0.5kg in the morning and and again in the evening. I use Pooltrine and Swimtrine to control the algae normally, so I gave it an extra dose of these, then backwashed the filter.

But the algae seems to be thriving.

Any ideas?

Yes you need to check for phosphates (Organics).

If it's high, dump some water to reduce it.

You did the normal thing but you are bashing your head against the wall if it's high.

Phosphates can be introduced many ways, rain, fertiliser, leaves etc. even via bodies entering the pool.

9 hours ago, 3STTW said:

Any ideas?

Yes.

Drain the pool.

Build a deck over it.

Remove deck when you want to sell to the next sucker who thought having a pool would be a good idea.

You're welcome. 😄

It sounded like chlorine level went down, as happened to myself before and my daughter's pool also, though not salt water, so I went to AI to have it explain it more.................Adding pH-down powder (sodium bisulfate) did not cause the algae; rather, your chlorine likely dropped below 1 ppm or your pH had crept high enough before the adjustment to let spores multiply. Salt chlorinators constantly push pH upward, and if sanitization lags, an outbreak hits fast.

How to Fix the Green Algae Bloom

• Brush and Clean: Scrub all pool walls, steps, and floor areas thoroughly to loosen the algae. Clean or backwash your pool filter, as it is likely full of green debris.

• Adjust pH: Test the water and ensure your pH is balanced between 7.2 and 7.4 so that your upcoming chlorine treatment works at maximum strength.

• Shock the Pool: Add a heavy dose of liquid chlorine or a strong pool shock at night. Use a double or triple dose depending on how dark green the water is.

• Run the Pump: Keep your pool filtration system running continuously (24/7) until the water turns a cloudy blue or gray, meaning the algae is dead.

• Vacuum and Clear: Vacuum any settled debris straight to waste (bypassing the filter), and use a pool clarifier if needed to polish the water clear.

Edited July 29Jul 29 by fredwiggy

14 hours ago, fredwiggy said:

It sounded like chlorine level went down, as happened to myself before and my daughter's pool also, though not salt water, so I went to AI to have it explain it more.................Adding pH-down powder (sodium bisulfate) did not cause the algae; rather, your chlorine likely dropped below 1 ppm or your pH had crept high enough before the adjustment to let spores multiply. Salt chlorinators constantly push pH upward, and if sanitization lags, an outbreak hits fast.

How to Fix the Green Algae Bloom

• Brush and Clean: Scrub all pool walls, steps, and floor areas thoroughly to loosen the algae. Clean or backwash your pool filter, as it is likely full of green debris.

• Adjust pH: Test the water and ensure your pH is balanced between 7.2 and 7.4 so that your upcoming chlorine treatment works at maximum strength.

• Shock the Pool: Add a heavy dose of liquid chlorine or a strong pool shock at night. Use a double or triple dose depending on how dark green the water is.

• Run the Pump: Keep your pool filtration system running continuously (24/7) until the water turns a cloudy blue or gray, meaning the algae is dead.

• Vacuum and Clear: Vacuum any settled debris straight to waste (bypassing the filter), and use a pool clarifier if needed to polish the water clear.

Yeah, that summarizes what I have already done - it's my usual algae treatment regime. Only this time it didn't work, hence the post.

14 hours ago, Bagwain said:

Yes you need to check for phosphates (Organics).

If it's high, dump some water to reduce it.

You did the normal thing but you are bashing your head against the wall if it's high.

Phosphates can be introduced many ways, rain, fertiliser, leaves etc. even via bodies entering the pool.

Yeah, this could be an issue. The house over the road just replaced all their water a couple of weeks ago.

After I laced it with a higher dose of algaecide yesterday, it has improved and the flocculant in the Pooltrine has dragged a lot of it to the bottom. A backwash this evening shows the filter is doing its job - thick green slurry after 24 hrs.

I need more shock powder, so I'll give it another hit tomorrow and see where we're at.

Some thing amiss here. Why was "Adding pH-down powder (sodium bisulfate) did not cause the algae" even brought into the conversation? Salt chlorination creates chlorine gas which in turn creates "Sodium Hypochlorite" and that pushes pH up. Acid is what is used to bring pH down. Bring (Soda Ash) "Sodium Bisulfate"into the equation only confuses things. It used to bring pH up with hand dosing of dry chlorine which is acidic. As you have a regular routine it highly possible that something had entered (Contaminated) the pool. Adding chemicals straight away with out knowing what caused it is asking for more problems. That is why I advised to check for Phosphates 1st.

Another tip - if it is a lot of algae vacuum to waste and not through the filter media. You will lose water but that will help!

Trust you get it sorted!

Edited July 30Jul 30 by Bagwain

I also have been fighting algae

No matter what I tried it wasn't helping

I chatted with a pool builder in isaan who went through everything that could cause the problem

I would see small light greenish dirt on the pool floor just on the grout area that I manually vacuumed every day

I changed to a better salt chlorinator and still after 2-3 weeks I needed to shock and floc

I tried a pool sock on the skimmer filter and this seems to be working

Not vacuumed for 3-4 days as still crystal clear water

It seems like the filter letting through microscopic dust/dirt particles that the pool sock now catches

Washing the pools sock out each day but it's getting much clearer and no algae yet almost 3 weeks so I am happy

Hate having had to vacuum each day so this is fantastic.

Never had any problems from October last year until around April when it went crazy hot.

On 7/29/2026 at 7:57 PM, 3STTW said:

I have a salt water pool and I balance the pH once per week. Last Wednesday I lobbed in the usual quantity of pH-down powder and the next day I had a nasty green algae bloom. I checked the pH and it was about 7.1, i.e. usual after treatment.

I then gave it a chlorine shock, about 0.5kg in the morning and and again in the evening. I use Pooltrine and Swimtrine to control the algae normally, so I gave it an extra dose of these, then backwashed the filter.

But the algae seems to be thriving.

Any ideas?

I have been going through the same problems and had to use floculant every 2-3 weeks

The levels were all fine and if anything chlorine was too high

For me a pool sock on the skimmer basket seems to have solved the problem

Clearest it has been for months

How old is the medium in the filter, perhaps its past its best.

If you are going to change the media you won't go wrong with the best there is, "Eco Clear" Recycled plate glass. I have attached the brochure. Been using it for well over 10 years with many many happy customers. Lasts up to 15 years. There are 2 types on the market. Don't buy the coloured one as it is bottle glass and doesn't work!

54 minutes ago, sometime said:

How old is the medium in the filter, perhaps its past its best.

It's only around 8 months old

Maybe I should change it

But it looks fine

9 minutes ago, Bagwain said:

If you are going to change the media you won't go wrong with the best there is, "Eco Clear" Recycled plate glass. I have attached the brochure. Been using it for well over 10 years with many many happy customers. Lasts up to 15 years. There are 2 types on the market. Don't buy the coloured one as it is bottle glass and doesn't work!

In my filter it looks like bottle glass but I I am sure it's just sand?

1 hour ago, kwak250 said:

In my filter it looks like bottle glass but I I am sure it's just sand?

I had bottled glass it gave me lots of trouble, Bagwain came to fit new valves & put plate glass in the filter,it's never looked so clear.

Where is your cyanuric acid, is it holding the CHL?

Apparently Trump has a guy that can handle algae in pools really well. Maybe you could get him to look at it.

7 hours ago, Priorexpat said:

Where is your cyanuric acid, is it holding the CHL?

Yes i have everything balanced

Chlorine is a little high but even then without the pool sock on the skimmer filter basket it was starting to look angry green quite quickly

Bagwain i am sure is correct with the plate glass media because i was seeing small amounts of greenish brown very fine dirt settle on the grout area every day but much less now with the pool sock

Edited August 8Aug 8 by kwak250

19 hours ago, Priorexpat said:

Where is your cyanuric acid, is it holding the CHL?

The op stated that he has salt chlorination so most likely there will be no residual cynuric acid. Cynuric acid becomes a problem in hand dose pools that use 90% Tri-Chlor If it gets over 50% ppm then it locks up the chlorines & water needs to be dumped.

Update from the OP. My algae is dead and the water is crystal clear again, I just have a lot of dead algae lying at the bottom. The robot has slurped most of it up, so the pool is just waiting for its indolent owner to get in there with the vacuum.

But the thing is, I had to triple dose the pool to knock the algae down, which begs the question - I pay $$$ for good quality algacide which has always worked in the past, are the suppliers now diluting the stuff - or is the algae becoming resistant?

Note, I buy the chems from a 'popular online pool specialist'.

12 minutes ago, 3STTW said:

Update from the OP. My algae is dead and the water is crystal clear again, I just have a lot of dead algae lying at the bottom. The robot has slurped most of it up, so the pool is just waiting for its indolent owner to get in there with the vacuum.

But the thing is, I had to triple dose the pool to knock the algae down, which begs the question - I pay $$$ for good quality algacide which has always worked in the past, are the suppliers now diluting the stuff - or is the algae becoming resistant?

Note, I buy the chems from a 'popular online pool specialist'.

I was told if your chlorine is balanced along with everything else then you shouldn't get algae but I definitely do.

I have been struggling to fight it and every 2-3 weeks I have used floculant .

From around October-March I never had a problem so I am sure the heat/rain and probably my filter containing bottle glass doesn't help

I have started using a decent clarifier so will see how that goes

I rarely brush the walls as hate doing it and never notice that they need cleaning

10 hours ago, 3STTW said:

Update from the OP. My algae is dead and the water is crystal clear again, I just have a lot of dead algae lying at the bottom. The robot has slurped most of it up, so the pool is just waiting for its indolent owner to get in there with the vacuum.

But the thing is, I had to triple dose the pool to knock the algae down, which begs the question - I pay $$$ for good quality algacide which has always worked in the past, are the suppliers now diluting the stuff - or is the algae becoming resistant?

Note, I buy the chems from a 'popular online pool specialist'.

You may have too much algaecide or you have chlorine lock. Which comes from high cynuric levels.

You only need "point 3 ppm" of copper for algaecide to work and it doesn't go anywhere accept for backwash & too much rain.

Copper is the main component of algaecide. Alternatively it could have been constant bad weather mucking up the water balance.

Be thankfull it is not blackspot or mustard algae.

On 7/30/2026 at 5:48 AM, SiSePuede419 said:

Yes.

Drain the pool.

Build a deck over it.

Remove deck when you want to sell to the next sucker who thought having a pool would be a good idea.

You're welcome. 😄

Even better, pool (in Pattaya) and all rquipment designed and installed by a qualified pool man.

Two years later I rented the house to a farang couple. Two months after they moved in the outspoken brit wife called me and said 'I need you at the house now'.

She had decided the electricity uage was way too high so she told her engineer husband to buy a new electric motor one quarter the size already installed. Hubby always did as he was told and bought a new electric motor and paid for it.

Tehnician was organised to install the but he refused to touch it without me being on site and approving the installation and the cost of the installation.

Brit renter lady attacked me 'you do as your told, I was the electric motor changed, today'.

I responded 'I will call the police and the HR dept. of your husband's work and report that you are behind in the rent'.

She retorted 'but you need to pay for the new electric motor regardless.' My Thai adult son intervened and called the village office / manager 9speks excellent English. He came to the house 2 minutes later and said 'ohh you again, He called a number then said 'police will be here in 5 minutes'.

Her husband's company got sick of the issues with both the husband and wife. Then I discovered she had paid 2 teen Thai boys to tear down the wallpaper in the living room.

Police went to the company office near Rayong and told the company, 'send both husband and wife home to England immediately after all the problems started by the wife are fully resolved. Within 2 weeks and with follow up by the police they flew back to England but only after they had given me (as instructed by the snr. police officer 100,000Baht to cover the isssues she had started.

Just now, scorecard said:

Even better, pool (in Pattaya) and all equipment designed and installed by a qualified pool man.

Two years later I rented the house to a farang couple. Two months after they moved in the outspoken brit wife called me and said 'I need you at the house now'.

She had decided the electricity usage was way too high so she told her engineer husband to buy a new electric motor one quarter the size already installed. Hubby always did as he was told and bought a new electric motor and paid for it.

Tehnician was organised to install the but he refused to touch it without me being on site and approving the installation and the cost of the installation.

Brit renter lady attacked me 'you do as your told, I want the electric motor changed, today'.

I responded 'I will call the police and the HR dept. of your husband's work and report that you are behind in the rent'.

She retorted 'but you need to pay for the new electric motor regardless.' My Thai adult son intervened and called the village office / manager (speaks excellent English). He came to the house 2 minutes later and said 'ohh you again, He called a number then said 'police will be here in 5 minutes'.

Her husband's company got sick of the issues with both the husband and wife. Then a further item I discovered she had paid 2 teen Thai boys to tear down the wallpaper in the living room.

Police went to the company office near Rayong and told the company, 'send both husband and wife home to England immediately after all the problems started by the wife are fully resolved. Within 2 weeks and with follow up by the police they flew back to England but only after they had given me (as instructed by the snr. police officer) 100,000Baht to cover the isssues she had started.

On 7/30/2026 at 4:18 AM, SiSePuede419 said:

Yes.

Drain the pool.

Build a deck over it.

Remove deck when you want to sell to the next sucker who thought having a pool would be a good idea.

When someone asks for advice about an issue they’re facing, this is obviously not the most helpful type of response. Forums like this are supposed to be useful places where people can share experiences and offer advice, especially from those who may have actually faced the same problem.

Of course, there are always those whose default contribution is the ever-helpful “Google it.” Then there are the experts who respond with sarcasm, despite having absolutely nothing useful to add... and, quite often, not even the slightest understanding of the question being asked.

If you don’t have the faintest idea about the issue, the simplest and most constructive option is to ignore the question and move on. Nobody is expecting everyone to have an answer.

Because an irrelevant, sarcastic response contributes absolutely nothing to the discussion. In fact, it often ends up being a rather accurate reflection of the person posting it.

10 hours ago, ravip said:

When someone asks for advice about an issue they’re facing, this is obviously not the most helpful type of response. Forums like this are supposed to be useful places where people can share experiences and offer advice, especially from those who may have actually faced the same problem.

Of course, there are always those whose default contribution is the ever-helpful “Google it.” Then there are the experts who respond with sarcasm, despite having absolutely nothing useful to add... and, quite often, not even the slightest understanding of the question being asked.

If you don’t have the faintest idea about the issue, the simplest and most constructive option is to ignore the question and move on. Nobody is expecting everyone to have an answer.

Because an irrelevant, sarcastic response contributes absolutely nothing to the discussion. In fact, it often ends up being a rather accurate reflection of the person posting it.

Their wife probably makes them do the dishes and clean the house

Let them have their fun before they get pegged again when the wife gets twated on 6 beer changs

I feel sorry for them.

Edited August 15Aug 15 by kwak250

On 8/12/2026 at 2:58 AM, scorecard said:

Even better, pool (in Pattaya) and all rquipment designed and installed by a qualified pool man.

Two years later I rented the house to a farang couple. Two months after they moved in the outspoken brit wife called me and said 'I need you at the house now'.

She had decided the electricity uage was way too high so she told her engineer husband to buy a new electric motor one quarter the size already installed. Hubby always did as he was told and bought a new electric motor and paid for it.

Tehnician was organised to install the but he refused to touch it without me being on site and approving the installation and the cost of the installation.

Brit renter lady attacked me 'you do as your told, I was the electric motor changed, today'.

I responded 'I will call the police and the HR dept. of your husband's work and report that you are behind in the rent'.

She retorted 'but you need to pay for the new electric motor regardless.' My Thai adult son intervened and called the village office / manager 9speks excellent English. He came to the house 2 minutes later and said 'ohh you again, He called a number then said 'police will be here in 5 minutes'.

Her husband's company got sick of the issues with both the husband and wife. Then I discovered she had paid 2 teen Thai boys to tear down the wallpaper in the living room.

Police went to the company office near Rayong and told the company, 'send both husband and wife home to England immediately after all the problems started by the wife are fully resolved. Within 2 weeks and with follow up by the police they flew back to England but only after they had given me (as instructed by the snr. police officer 100,000Baht to cover the isssues she had started.

Not sure how much truth was in that story but having a oversize pump can have problems itself.

The electric bill is the least of the problems .

On 8/10/2026 at 9:02 PM, kwak250 said:

I was told if your chlorine is balanced along with everything else then you shouldn't get algae but I definitely do.

Depends on which chlorine you measure, total chlorine or free chlorine. Those tubes which you add a yellow and red liquid measure total chlorine, which means nothing.

I have a saltwater pool for 14 years. I measure salt and Ph with a digital meter, and chlorine with test strips, and I can't remember last time I added chemicals.

I top up with well water which has a Ph of about 4. The only place where I sometimes see green algae is at on the sandwash edge at the side gets sun all day. I just spray it with bleach when I see it. Never had algae in the water.