Three weeks after fill and my ch is 620.

Is the only recourse a partial drain and fill? Should I wait a while until curing is more complete?

That is dramatic, much more than we usually see. What is your fill water CH level?

The two possible solutions are drain and refill or a reverse osmosis treatment, though I have no idea if RO is available in your area.

Geez and I just finished getting the TA and cya where I wanted them. I will test the fill water and post results

Fill water is 100. Double checked pool and it's definitely 620. Why would this happen?

Pretty sure RO is not available around here, water is plentiful and cheap.

You have been around here a while, so I wouldn't expect this to be an issue, but in situations like this one it is always a good idea to double check your test procedure, to make sure you aren't doing the CH test wrong.

As plaster cures it raises the PH, TA, and CH levels. Just how much that raises the CH level over the first month is unpredictable, but typically is around 100-300 ppm. You are above that, but not completely outside the range of plausible. The other possibility is that the PH was a bit too low for a bit too long at some point and that dissolved extra calcium out of the surface and into the water.

I think that is what happened.

12 gallons of 31.45 % acid in 16,000 gallons of water will lower the TA by about 380 ppm. So, subtract 380 ppm from your fill water TA to get what your TA was. It was probably strongly negative unless the TA was above 380 to begin with.

The pH would have been well below 4.5.

This is called an acid startup. However, 12 gallons of acid in 16,000 gallons of water is very aggressive unless the acid is a lot lower strength than 31.45 %.

All of that acid would have dissolved a lot of calcium out of the plaster.

Also, if the system was online during the low pH, then there might be copper in the water from the heat exchanger, which could interfere with the calcium hardness reading.

Your posts indicate that there are metals in the water. The green water was probably from copper. Try this modified test to eliminate copper interference:

Make a sample of 50 % pool water and 50 % distilled water. Mix well. Follow these steps to test:

1) Rinse and fill large comparator tube to 25 mL mark with water to be tested.

2) Add 10 drops R-0012. Swirl to mix.

3) Add 20 drops R-0010. Swirl to mix.

4) Add 5 drops R-0011L. Swirl to mix. If calcium hardness is present, sample will turn red.

5) Add R-0012 dropwise. After each drop, count and swirl to mix until color changes from red to blue.

6) Add the original 10 drops of R-0012 to the number of drops needed in step 5, and multiply the total number of drops by 20. Record as parts per million (ppm) calcium hardness as calcium carbonate.

Thanks guys. Will do the modified test, then drain and fill again in a cpl weeks.

JamesW why the pool/distilled water mix?

Using distilled water will cut the metal level in half, which will reduce the interference. It also reduces the calcium level by 50 %, so that's why the number of drops is multiplied by 20 instead of the normal 10.

I think that the acid will account for some of the calcium, but I don't think that it can account for all of it.

Theoretically, the acid could only account for about 380 ppm of the calcium if the acid was completely neutralized by calcium hydroxide or calcium carbonate. Since you had to add washing soda and baking soda to raise the pH and alkalinity, then all of the acid must not have been neutralized.

If the fill water is low in alkalinity, then that could account for part of it, but still not enough to account for all of the calcium. If you haven't added any calcium, then I would suspect the rest is due to test error due to copper.

If you can have the water tested for copper, that would help.

I agree with JasonLion regarding the test procedures. I just retested my CH last week (I do it once a month) and I got 500 which was down from 600 the month before. Nothing happened to cause a reduction in CH in my pool during the month. I have been using a Taylor SpeedStir, but this time I paused a full 3 seconds (with the SpeedStir running, of course) between each drop. It made a difference. And the SpeedStir really makes a difference. I was getting crazy results before I bought the SpeedStir.