My new plaster pool is COVERED in green stain..

[hausfrau]

We added the muriatic acid to the water this time...rather than draining and spraying. It is snow white again.

[waterbear]

Muriatic acid is not killing the algae. it is removing a layer of plaster from your pool and that is not always a good thing.
Extended periods of pH below 7.0 are hard on pool equipment at best and outright damaging at worst.

If you have a heater I hope you bypassed it because heat exchangers are easily damaged by low pH. The result of damage to a heat exchanger is usually green hair and copper stains on the plaster--out of the frying pan and into the fire so to speak.

There are methods for doing a "no drain acid wash" which is what you have attempted but they include monitoring the pH and bringing the pH and TA back up once the treatment is finished. They should not be attempted unless you have both acid and base demand tests and access to the large amounts of baking soda, soda ash, and/or borax that is going to be needed to stop the process. Also, a pH testing method other than phenol red is a good idea so you know just how low the pH really is. Best choice is a calibrated pH meter (expensive piece of equipment).

While you may have removed the stain you have not killed the algae.
to answer your questions as you numbered them:

1. you do not want the pH below 7.2 for an extended period of time! Amount of acid that will lower the pH depends on your starting pH and TA so it is impossible to give a set amount.

2.In a word, no.

3. Algae is not going to be your problem now. You have probably elevated your calcium hardness at this point by dissolving plaster. Just clean the filter like you normally do.

4. Muriatic acid is used to remove metal stains when other methods fail. Certain copper stains (the black ones) are very difficult to remove and acid washing is often the only recourse. Realize that it removes the stain by removing a layer of plaster. You can only do this so often before you need to re-plaster the pool.

Also, acid washing is the only thing that can remove scale (calcium deposits). Realize that calcium is also a metal.

Finally, Muriatic acid does not kill algae, chlorine does. If you had a bit of patience and persistence you would have eliminated the problem without causing damage to your plaster.

Hope this is helpful and gives you something to think about.

[sunofthebeach]

do you have any of the"algae" left? Can you post a picture?

It is hard for me to believe your pool installer did not know what algae looks like.

I do like your style hausfrau, charge forward and try anything and everything.

If it were me I would go out today and get a test kit, even if it is a $5 chlorine and PH kit from Walmart. I have rough, pitted in some areas plaster because I did not pay enough attention to the PH and Calcium.

If what you really have is Algea, you are going about it wrong goofing with the muratic acid. I am sure acid will kill algea, but you probably harming something else.

[hausfrau]

Waterbear & sunofthebeach. Thanks for your posts. I should clarify. What killed the algea in the spa was not the acid but a shock to the spa. I isolated the spa from the pool and elevated the Cl enough to kill it then lowered the PH level to 6.5 for 4 hours. Not sure but I believe that the lower PH and short time period should not have attacked the plaster agressively. After the soak I purged the Spa. The spa feeds the pool. (500 gal into 14400 gal). I did not drain from the spa only from the pool so 6.5 PH water never touched the heater. Test strip shows Ca at normal levels for our water after 16 hours of circulation. Testing for Cu in both pool and supply show very low levels.

After the stain removal step. I circulated the pump for three days and the algea returned leading me to believe that a high sustained Cl level is needed to truly rid the pool of algea. As for the staining more than likely we will see what happens after a week or so. I purchased a test kit to get a better testing capability than the strips.

per an earlier post i read a sustained level of 25ppm is needed for shock levels. Any ideas on time period?

[aylad]

Maintain shock levels until the pool clears, the CC goes to less than 0.5, and you're not losing any chlorine when comparing test results from testing at sundown and again in the morning before sun hits the pool.

Janet