I am the new owner of swimming pool for four and half months. The pool was excellent when we moved. The FC was 4 ppm, only pH was high around 8.2. I added MA to reduce pH, while pH was not lowered due to high alkalinity.

I found TFP website in the early of July and knew the BBB method. I stopped adding 3" tablet and switched to bleach. FC was 0 before I switched to bleach. 4 gallon of MA was added to lower alkalinity (around 80-100; used to be 190) and pH (7.2-7.5). However, pH jumped back to 8.2 a few days later and alkalinity climbs slowly. I use the test kit recommended in this website and test results showed that pH of tapping water is 8.2 and alkalinity is high in tapping water.

Since the pH is not well adjusted and bleach does not work effectively, green and black algae are growing at some spots in swimming pool. Do I need to add MA quite often to lower alkalinity and pH? Is that OK to leave the pool with high pH? TDS jumped to 4100 ppm because around 6 gallon of MA was added during these 4.5 months. Do I need to replace the water? I need to shock the pool right now?

Welcome to TFP!

You must always maintain pH and the proper FC for your CYA level. That's how the pool stays Trouble-free.

You should run the full set of tests and post the results in this thread. Armed with the facts, and input from other people, you can then decide if you need to drain. If you have algae, shocking is a foregone conclusion. Whether you can use cal-hypo, di-chlor, or bleach to shock can't be determined unless we have the full set of results.

After results are posted, head on over to Pool School for a refresher course. Read especially the articles on pool chemistry, algae, and shocking.

Water replacement is sometimes an option depending on the type and size of pool. If algea are growing, escpecially if you can see them, then you need to do something sooner than later. Read the articles (pool school) on dealing with algea. Post those test results (esp. current FC, CYA and pH) , and I am sure you will receive detailed help from others on the shocking process. I am going to assume shocking is your best option for now. pH rise, while troublesome, takes a backseat to the algea in my opinion, so I would tackle that first.

Thank you for all your reply. I will post test results tonight.

I do not know the details of equipment. Do I need rent any equipment to drain the pool? After reading the posts, I think drain 1/3 is safe.

Again, depends on the pool type. Above ground, you likely only need a hose, and gravity will do the work. In ground, you will likely need a pump. In either case, you likely already have a pump connected to the pool, so you would not need to rent one.

If you can post pool type and approximate volume, that would be helpful. Pictures work well too. I did not really understand much of my pools equipment either, I posted pictures and now I understand how it works.

Still, I am not sure partial draining will be helpful. You can add fresh water after draining, but the algea will still be in there and just continue to grow. You have already established that "fresh water" in your case is high in pH and TA, which you have been already been working at lowering. So it seems like you would be creating more work doing that.

Getting your pH to around 7.2 and then shocking is what I would recommend (and is what I am currently working on with good results in my own pool).

It is in gound pool with 26,000 gallon, cartridge filter. There is a pump running around 12 hours per day to circulate the water. The pump can be used to drain the pool?

Test results:
FC:<0.5
TA:160
CH:340
pH:>8.2
CYA:40-50
TDS:4000-4200 ppm, read from control panel

Tapping water:
pH: >8.2
TA: 210

Even tho you did not post a CC level, the fact that you have green and black algae means that you need to shock.
But, first lower that pH to about 7.2. Use the PoolCalculator.Com to figure the dose.

Once you lower the pH, start the shock process - directions in Pool School.

You don't need to drain your pool.

Post back in this thread with any questions.

Can you post some pics of the algae?

Welcome to the forum

Lowering pH means adding a lot of Cl-, increasing TDS, which is already very high in my pool. There is error message for high salt on control panel everyday.

TDS was 3500 ppm when we moved in; then it jumped to 4000 TDS. I heard the best value for TDS in swimming pool is 1000-2000 ppm. Can anyone confirm this number?
In general, the higher salt concentration, the easier or the faster for algae to grown since there are a lot of nutrients for them.

How important the pH is for the effectiveness for chlorine? Is there a chart to show that?

Lowering pH means adding Acid. Not sure how this affects TDS, I have never measured TDS and I do not think it is necessary to know TDS.

High Salt is a different story. DO you have a test for salt? High salt would be a reason to do the partial drain you wanted. Still, until the algea is under control, this is probably a secondary concern for you.

the pump is pumping pool water through the filter (outside the pool) and then sending the water back into the pool. There may be a valve, probably located prior to the filter, to redirect the water to the ground. Again, you probably do not need to drain the pool at all at this point, unless you verify that salt level is too high.

FWIW our tap water here is adjusted "to" 8.0-8.2 so it is less corrosive to metal (according to the Water Bureau's website). Not only the metal piping, but all hydronic machinery and fittings/valves are constructed out of metal components (pumps/boilers/coil units made from steel, brass, copper, etc.). I would expect this is fairly standard procedure in most municipalities, regardless of location.

Adding acid increases chloride salt (if using Muriatic Acid; if using dry acid then it increases sulfates). 1 gallon of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in 10,000 gallons would increase salt (measured as ppm sodium chloride) by around 58 ppm.

You are right. I totally added 6 gallon of MA to the pool during the past 4 months, while the TDS should only increase around 100 ppm.

There isn't a best value for TDS in a swimming pool unless a saltwater chlorine generator is used in which case they are designed to operate in a certain range. They need a minimum salt level to generate chlorine efficiently as opposed to generating mostly oxygen gas. If the salt level gets too high, some systems generate too quickly with too much current, though other systems are self-regulating. The usual salt level for chlorine generators sold in the U.S. is around 3000 ppm.

It is not true that algae grow more easily with higher TDS. TDS is mostly salt, not nutrients for algae. In spite of plentiful algae nutrients (such as phosphates and nitrates), algae are ultimately limited in their growth by sunlight and temperature so chlorine (at appropriate FC/CYA levels) is able to kill algae faster than it can grow.

As for the pH dependence with active chlorine level, with CYA in the water the pH has less of an effect so you should mostly just have the pH in the range that is most comfortable to the eyes, doesn't corrode equipment and doesn't encourage metal staining or scaling. The proper pH is usually in the 7.2 to 7.8 range. This post shows the active chlorine level vs. pH with and without CYA in the water.