Clorine Block - Need Advice

I have a 20,000 gallon vinyl liner in ground pool using a sand filter. About a week ago the water turned green. We have tried multiple applications of algicide. The green color has gone. Now the water is cloudy. We can see the bottom pattern at the shallow end, but not the deep end. Over the past 5 days we have, on the first 4, done repeated 8 pound shock treatments each day. Water is now blue, as indicated above. Numbers are as follows this morning:
the ph was 6.8 alkalinity 140 or 150. Cyan stabilizer was 100. Chlorine was 1 but it measured total chlorine. These measurements were done using a pool kit which did not offer a test for free chlorine. We tested with at strip for FC and it was about .5. The strip was slightly lavender at the edge of the sponge so free chlorine might be .5. It wasn’t completely colorless but close. The whole sponge was lavender the morning after the last 8 pack shock. It was definitely 1 in free chlorine then.

I have a large quantity (about 16 gallons) of 6% Chlorine Bleach availalbe. Question is, how much should I use? Any other ideas?

I forgot to mention - I have a Nature II unit if that makes a difference. Thanks.

hello Kevin, and welcome to the forum.

First off, "chlorine block" doesn't exist. It's often a way for pool stores to try to give answers about pool problems when they're not sure of the problem. When you have an algae bloom, you need to take the chlorine up to "shock" level, which in your case is 25 ppm, and hold it there until the pool clears by testing and adding more chlorine as many times a day as possible. In your case, if the pool has turned blue, you may be through shocking and may not--you can tell by testing at night after the sun is off the water and then testing again in the morning before the sun hits the pool--if you've lost more than 1 ppm chlorine, then you're not finished yet, and you need to get the chlorine back up to 25 ppm and hold it there until you don't lose chlorine overnight. Since your pH is so low and your CYa is so high, I'm guessing that you're using trichlor tabs to chlorinate and maybe dichlor powder to shock with? You need to stop using any stabilized form of chlorine at this point, because your CYA is so high that you're going to have to run a higher baseline chlorine level to compensate in order to keep algae away. You can check out the "best guess chart" in my sig, but with a CYA of 100, this means that after the shocking process is finished and the pool is cleared up, you must keep your chlorine in the 8-15 ppm range at all times to keep from getting another algae bloom. The higher the CYA, the higher the chlorine levels you must keep.

You also need to get that pH up above 7.0 as soon as you can--anything below 7.0 is acidic and can damage your pool liner. We suggest Borax for this--added through the skimmer very slowly, breaking up clumps, allow a couple of hours to circulate and then test again, adding more if necessary. You want to target 7.2-7.6. I would start with 1/2 box in your case, and go from there. Do this before taking your chlorine back up, though, because chlorine above 10 ppm can cause falsly high pH readings.

In a 20K gallon pool, each 1.5 quarts of 6% bleach will raise your FC by 1 ppm, and each 1.5 gallons will raise your FC by 4.5 ppm, so you can use that as a guide when figuring your bleach.

We don't really recommend the Nature II units--I know they advertise that you can run lower levels of chlorine with them, but that is very difficult to do without an algae bloom, and they do add copper to the pool, which results in staining the pool, blonde hair, fingernails, etc green. You can use the google link my my sig to look around the forum for other posts on the Nature II.

Janet

Hi Kevin. Jan is giving you excellent advice. Another way to estimate how much bleach to use is that with 0 FC, you'll need 8 1/3 gallons as each gallon (they don't sell it in gallons anymore, do they?) will add 3ppm of FC.

Or you can use 12.5% liquid chlorine. If the per gallon price is less than double the per gallon price of 6% bleach, it's a good price, because you use half as much.

Having been in your situation (at least the low pH part) in a 20K pool, same size, I would be a little more aggressive than Jan with the first dose of Borax and use a whole box. It may be less than 6.8, but at 6.8, in 20k gallons, 1 box of Borax won't overshoot your ideal pH target. Again, I've been there a couple of times and it will EASILY take one box. The second box, if needed, go "halfsies".

Carl

Thanks very much Janet and Carl. We are going to work on it tonight. Will let you know the results.
Kevin

Well, when I got home last night, the Ph was up a bit - to almost 7.0 from 6.8. We put in 1/2 box of borax and that raised the Ph to 7.2 or so and that held overnight. The Cya last night appeared to be 85-90 instead of 100. We are having difficulty reading this - seems somewhat arbitrary based on who is doing the reading. Should the black dot disappear completely? If it is bearly visible is the reading finished? We also discovered we have a problem with the pool test kit we are using. It only measures up to 5 ppm. I have ordered the kit recommended on this site through amazon - but it will be about another week before we get it. Anyway, we added 6 (182) oz containers of bleach last night - which would amount to about 20 oz more than the recommended 8.3 gallons. The water immediately turned from blue to green. But by this morning, it was blue again, with a reading of 3 ppm TC (by drop kit) and 2 FC (by strip - only method we have right now of measuring FC). Pool was heavily brushed and vacummed this morning. We did add the mfg recommended dose of Clarifier to try to help clear the dead algae. We are waiting to see what happens when we get home tonight to the readings. Any thoughts?

Don't add anything thing else right now but bleach. No clarifier, no other pool store goop. With a CYA of 85-90 or even 100, I'd suggest shocking up to 20-25ppm. Each time you test, add enough bleach to get back to 20-25ppm. No such thing as testing and shocking too often when you are trying to clear a pool. You should also be running your pump 24/7 and cleaning the filter as needed.

Regarding the CYA test --- you read it when the black dot completely disappears. It definitely is the hardest test to read.

Until your kit comes, you can use a dilution method to force your kit to go higher. You will need to buy a jug of distilled water to use to dilute your water sample with. More info about it can be found here: Testing Without a Good Kit

Hope this helps!

agreed, keep things balanced