Does this look like a stain from metals?
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As your CyA is very high it could be causing real imbalance issues DRAIN THE POOL GET IT TO IDEAL LEVELS AND THEN START. PLEASE READ ENTIRE MESSAGE TWICE TO UNDERSTAND. These are instructions for a plaster pool.
The fact that the Vitamin C tablet caused a charcoal color could mean that this is algae and not metal stains. Try dropping a Tri-Chlor tablet on it for 24 hours. Or if you were using anything other than a WHITE vitamin C tablet the other ingredients caused the charcoal color, especially if it is a brown tablet. If it is algae, once you have your water at ideal numbers shock it at 35 -40 ppm chlorine, keeping it at these numbers for 4 - 7 days, pump on 24/7. It is also a good idea to add Algaecide 60 to help it along.
To know if you are at the 35 - 40 ppm take 1 oz pool water add 8 oz distilled water mix and put into your test kit tube to the right level. A measurement of 5 would mean 5 x 8 or 40 ppm. A measurement of 4 would mean 4 x 8 or 32 ppm, so add enough liquid chlorine to get it up by 8 to 40 ppm. Once the numbers hold overnight eg. 40 at 7 pm and 40 early in the morning you should have killed all the algae. Then you can let it lower itself through sunlight.
If you want to take pH readings first add 6-8 drops of Thiosulfate used in the alkalinity test to neutralize the chlorine or you will get a false reading on the pH. If you do add a lot of chlorine the pH will rise so you most probably will need to add acid.
Firstly if you do not have a pool sweeper with brushes you will need to do a lot of brushing while all this is going on, preferably with a stainless steel brush. You might still need a stainless brush anyway to break it up. Stainless steel brushing cause lots of cloudiness so it is best to do this first and then backwash before adding maximum chlorine. Use only liquid chlorine either Clorox REGULAR ONLY or Wal-Mart's Ultra Bleach REGULAR ONLY not scented, etc. Chlorine is stronger at lower temperatures. Get Ben's calculator available from this forum. Use Baking Soda to adjust the alkalinity.
If after doing this treatment and the chlorine is stable at 40 ppm and the algae is still there you might have mustard algae which is resistant to chlorine. Then add 5 oz (per 10,000 gals) of Yellow Treat from United Chemical and let it do its stuff. The high chlorine will get used up quickly but it should kill the algae. When the chlorine stabilizes at 30, you can stop adding chlorine and let it drop to normal levels.
IF THESE ARE STAINS:
VERY VERY important drop the chlorine to 0, yes zero, before putting the the vitamin c (ascorbic acid) into the pool, or all the the acid will do is eat up the chlorine and not do the work on the stain. You can buy a chlorine reducing chemical from your local pool store. Sprinkle the powder around the walls or near the stains.
You can buy vitamin C POWDER from your local health store DO NOT GET the crystals they will not work as they are non-acidic.
You will need 1 to 1 1/2 lbs per 10,000 gals, more is better. Keep the pH low, say 7.0 and sprinkle it around the wall. I did mine at a pH of 6.0 in a gunite pool and it worked very well but a number of persons on this website think that 6.0 is far too low, although ascorbic acid has a pH 6.0; at 7.0 I had limited success. Never lower than 7.2 for a fiberglas pool.
If you are worried about an algae bloom put in one quart of Algaeside 60, not 50, 40 or 30 per 10,000 gals. Then immediately add 3 quarts per 10,000 gals of metal out or stain treat (not stain removal) to catch the metal stain in solution.
In regards to a sand filter adding DE helped catch more particles, then again this may be more in the mind than in reality. With a 300 lb sand filter (9,000 gal pool) add enough DE, through the skimmer box, to raise the pressure by 1, this is about 3 cups, add one cup at a time and wait 15 to 20 minutes. Once you have started the ascorbic acid process you may notice, if you have a sand or small filter, the pressure rising dramatically and your filter slowing down. I back washed for 1 minute only after each of the first two 12 hour periods as the filter became blocked.
It is best to first backwash and then COMPLETELY DRAIN the pool after having run the pump for 48 hours non-stop. If you do not drain the metal will eventually fall out of solution and cause the stains to reappear via deposit.
Refill and bring the calcium hardness up to at least 300 - 350 for gunite, 100 to 150 for vinyl. I think the higher number works better with stain problems although I am not sure why. Some members have suggested that calcium in the filter helps get the metals deposited in the filter and then washed out in the backwash.
Once you refill it is a good idea to shock your pool first with liquid bleach 20 ppm (Ultra Bleach from Walmart is cheap) at a low pH of 7.2 then after 48 hours bring your pool back into balance and FINALLY add 1 quart per 10,000 gals metal out to keep any metal introduced into pool with the new water in solution. Do this once a month, or as Mbar suggests a little say 3-4oz if the stains reappear.
DO NOT SHOCK if you are not refilling the pool. However if you do not drain and refill you will have the ascorbic acid in solution which will eat the chlorine in large doses. It will take lots and lots of chlorine before you can maintain a stable level. This may take 2-5 days of constantly adding chlorine, a real pain. Also as stated above the metal will restain the walls.
If you keep the pH between 7.4 - 7.5 (it keeps the metals in solution) you should not have too many stain problems. Keep the REAL alkalinity between 65 and 90 for gunite, 100 to 120 for vinyl, this also helps keep the metals from falling out of solution, using your CYA number to adjust this. Example a CYA of 50 X ph 7.4 adjustment of .30 gives a reading of 15.0. So your alkalinity should read 100 to get a real alkalinity of 85.0. At a CYA of 40 the adjustment is 40 X .30 or 12.0 so your reading should be around 110 to get a real count of 98.0.
ph CyF adjustment
6.5 0.11
7.0 0.22
7.2 0.26
7.4 0.30
7.6 0.33
7.8 0.35
8.0 0.36
8.5 0.38
Preferred CyA number is 50 - 55 ppm
If your CYA is 50 then you should keep the Total Chlorine between 6-4. Remember it takes 3-7 days for CYA reading to measure correctly as it dissolves very slowly, so do not add more than is necessary.
Do not shock the pool after you are in balance for at least 2 weeks or it may go cloudy. Add Algaecide 60 if you like.
I learnt most of this from Mbar so you have her to thank.
If you have blackish looking stain streaks these are normally caused by tannin from leaves, to dissolve these you would need to keep the chlorine up at around 35 or 40 ppm for 48 to 72 hours. Carefully monitor your pH level to 7.2 and Your REAL alkalinity to 70-90 for gunite, 100 to 120 for vinyl. Running your pump the entire time.
You may still need to do the the muriatic clean process but this is more difficult and should only be used as a last resort, or if your pool walls look stained after draining. One part muriatic to 2/3 parts water poured down the walls and when the fizzing has stopped spray with hose (ALWAYS ADD MURIATIC TO WATER OR IT COULD EXPLODE AND CAUSE BLINDNESS AND BURNS). If you do this it is a good idea to add plenty of soda ash to the bottom of the pool to neutralize the acid or it might eat out the bottom of your pool. Running a hose at the bottom of the pool is also another good safety measure.
I think doing the ascorbic acid process at a pH of 7.0 is better and less dangerous than putting a muriatic mix of say 1 part acid to 3 or 4 parts water directly onto the wall but once again some members may disagree and I have no experience of vinyl pools.
MY Ideal Levels Plaster pool (not everyones):
Alkalinity 65-85 (CyA adjusted)
pH 7.4 - 7.5 (Use Muriatic Acid to lower)
Calcium 300-350
CyA 50
Chlorine 8 max, 6 ideal / 4 minimum (2 and you will get algae) - Use only liquid chlorine not tabs.
Lastly adding Borax to my pool has helped a lot in contolling the pH bounce, it has also helped with stains, the instructions on how to do this are on this website. Do a search under Borax.
Aloha and good luck.
Ohmygosh. That was so confusing.
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