Low pH and High Alkalinity - Which Do I Fix First?

by Diane

After 15 yrs of pool ownership I'm having a horrible time with my pool. Out of the blue I've developed large black algae spots on my steps and bottom of pool. Has been very difficult to keep chlorine level up even though I shock my pool weekly (20,000 gal).

The pH has been holding steady at 7.2. Alkalinity has always been perfect but when I checked it recently level was 220 ppm. I have no idea how it got so high. I had green algae prior to black algae but able to get rid of it with supershock and algaecide. Haven't had my filter cleaned yet as waiting to get rid of black algae somehow.

Pool store recommended high copper product but you state not to use copper. Is Algi-ban better? Why not copper product? Should I try to lower my alkalinity first with muriatic acid or raise pH first with sodium bicarb?

I'm afraid of making alkalinity worse with sodium bicarb and lowering pH even further with muriatic acid. Do I correct one completely or alternate in adding acid, then bicarb, then acid, etc.

I've read that it's impossible to completely rid pool of black algae. Is that true? Is it dangerous to vacuum spa that flows via waterfall into pool? I'm afraid of getting air into line when I lift vacuum up and into spa. Thanks for your help! Have been afraid to swim in pool.

Thanks for the question Diane

I'd like to have your CYA reading. This can determine the right action to take. The range for CYA is 30 - 50ppm. Once it starts getting to 80 - 90ppm or above it will be time for a water change. There's no chemical to reduce CYA. A high CYA could be the reason why the chlorine is not holding. The relationship between chlorine and CYA is simple. The CYA is 7.5% of your chlorine. 2.5 ppm chlorine divided by 7.5% = 33 CYA. As the CYA rises the chlorine must rise as well to keep the relationship. If you have CYA of 90ppm, it means you need to run the chlorine at 7ppm. 7(chlorine divided by 7.5% = 93 CYA.

I really like these kinds of questions because there's lots of stuff going on. I'll first start with the total alkalinity (TA). You can refer to this post as well:

Why Does My Alkalinity Keep Going Up?

The TA can't go up on it's own. There must be a reason and most of the time it's either adding too much bicarb or the fill water's TA has risen. Are you on well or city water? If city, you can call them and ask if they've raised the TA. If they have, you have your answer.

There are two ways to lower the TA. First is to add acid to the perimeter of the pool with the pump on. This will lower both the pH and TA. Then you aerate (add air and bubbles) the pool using the slide, water fall, or air compressor. Attach a hose to it and put it in the pool. This will cause the pH to rise but not touch the TA. This way you can
alternate lowering the TA and raise the pH.

Next is to turn the pump motor OFF. Add the acid to the deep end in one spot, sweep the area very gently to break up any spots of acid on the floor, wait 3 - 4 hours, turn the pump back ON and allow to FILTER for 8 - 10 hours, then retest the TA and pH. This is the technique I prefer. Both techniques take time, persistence, and lots of patience. It's 1.5 qrts. of acid to lower the TA 10ppm per 20,000 gallons. The range for TA is 80 - 100ppm. You'll need approx. 4.5 gallons of acid to go from 220ppm to 100 - 120ppm. Don't do this all at once. Stages of 1/3 would be good for now.

Next, you need to know if you have black algae. Get a chlorine tablet and put it on the black spot for about 3 - 5 minutes. If it fades it's black algae. If it doesn't fade or gets darker it might be metal stains. Get some ascorbic acid (crushed up Vitamin C) put that in a sock, about 1/2 lb. and place it on a different black area. If it fades it's metal stains and you'll need to do an ascorbic acid (AA) treatment. We'll cross that bridge when we come to it.

Why NOT to use a copper algaecide? It's very simple. It has copper in it. Eventually you'll get metal stains due to constant use. And it can cause unwanted foaming. Use a PolyQuat 60 algaecide. Yes the copper is a bit cheaper for now but the bad you'll do far outweighs the good, if any, it does right now.

Algi-ban, and I assume this is HASA Algi-ban, is Trichloro-s-triazinetrione, which is a chlorine tablet. Have ever heard of Trichlor tabs? You'll be using nothing more than powered chlorine tablets. If your CYA is already high and you start using this stuff, it's going to go through the roof. You'll now have way too much CYA and the chances of getting green water is pretty much guaranteed.

If the same water is used for the pool and spa the chances of getting black algae in the spa is very high. And yes you don't want to lift the vacuum out of the water. What you can do is put the vacuum in the spa then put the other end of the hose in front of the pool return and fill it with water, then walk it over to the skimmer.

Get me the CYA reading and we'll go from there.

To post a reply, or if you have a similar question, you can see your post on the Q&A page in the "Alkalinity" category.

Check back to this post for updates or answers.

If you feel your situation is complex and want immediate assistance (within 24 hrs) you can make a donation of $35 per hour and I'll answer your questions by phone.

Have I helped you find a solution to your pool problems? Did this information help you? Please consider making a donation to help keep this site going. Thank you.

Hope this helps and have a great Summer.


Click here to post comments

Return to Total Alkalinity.

Total Alkalinity vs Adj. Total Alkalinity

by Rick
(Chicago, IL)

I had my water tested at a pool store. The Total alkalinity is 136 (a little high) but it says the Adjusted total alkalinity is 96 (they say a little low). They are recommending adding 22 lbs of "Balance Pak 100".

30,000 gallons
CYA 134 (I know, I just moved in and they've been using tirchlor tabs for all chlorine)
Total Chlorine 13.7
Free Chlorine 13.7
PH 7.6
Hardness 145
Tot Alk 136
Adj. Tot Alk 96

Thanks for the question Rick, and for the readings. It makes life so much easier. I've really never put much emphasis on the TA vs. the adjusted TA. Basically it's this. The CYA (stabilizer) in the pool forms another kind of buffer, or cyanurates. The CYA is taken into consideration when testing the TA (bicarb). A mathematical equation is used that is determined by the pH and CYA level. That number is then subtracted from the TA to get the adjusted TA. It's pretty confusing and not part of any pools that I've cleared up. I simply go on the TA, along with the other numbers.

Being that you say "I know" about the CYA, you probably know it's way too high. The normal range is 30 - 50ppm so you'll need to do a nearly full drain and refill to get it down to 30ppm. I guarantee that if you go the pool store route and add in the Balance Pak 100 (which is nothing more than baking soda) you'll walk out with about $300 worth of products and your pool will not be balanced and clear. You can get a 13 lb. bag at Costco for $8 or go to the pool store and buy 1 lb. for $12.99.

Do a drain/refill to get the CYA to 30ppm. Then drop the pH to 7.0 and use 4 gallons of chlorine, once in the AM and PM. Eight gallons in all. Sweep well and be sure the filter is running perfectly. This should kill any residual bacteria and algae in the pool. You should be set after that. If the TA is a bit low, add bicarb. to bring it up to the range of 80 - 120ppm. You should be set after that and it won't cost $300 in chemicals.

If you feel your situation is more complex than this, I do phone and/or SKYPE consultations. It makes things go much faster and many people have found it extremely beneficial, saving them time and money in the long run. All your questions will be answered. I have nothing to sell you so you know I'm not bias. If you purchase a personal phone consult you'll get all 3 eBooks for free.

Pool Consultation

Clear Blue Pool eBook

How To Clear Up A Green Pool eBook

Swimming Pool Resources


Click here to post comments

Return to Total Alkalinity.

eBooks Are Perfect Desktop & Mobile Device!!

swimming pool care, basic pool care, above ground pool maintenance, inground, salt water

Click Here To Learn How This eBook Can Save You TONS Of Time & Money


Click Here To Learn The Best Way To Clear Up Your Pool 


All Resources In One Convenient Package. Click HERE To Get Started.

Recent Articles

  1. How To Clear Up A Cloudy Pool eBook

    How To Clear Up A Cloudy Pool eBook

    Read More

  2. Pool Float & Sweep

    How do I keep the hose from my pool sweep and the floater from colliding? If I tie the float off where is the best place, and one last if the float is tied off.

    Read More

  3. The calcium hydochlorite will not dissolve in our pool

    The calcium hydochlorite will not dissolve in our pool. It is suspended in the water

    Read More