Low Chlorine

Causes and cures for cloudy swimming pool water.
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Low Chlorine

Postby lcjv95 » Mon 23 Jul, 2007 15:45

My water is not perfectly clear and I can't keep chlorine in the pool. I recently had to use an algecide to clear up "yellow algae" per pool specialist and I followed up with a really strong shock that had a clarifier as well as a stablizer for the chlorine. I have a 20000 gal. pool and I used 5lbs of this shock over the period of 2 days only to have the chlorine down again the next day and still cloudy. I also use about 5 chlorine tablets in the the basket in the pool at a time. My recent readings

0 free chlorine
7.5 ph
90 TA
50 CYA
1500 Total Dissolved solids
75 phosphates

What should I try next ? They recommended more CYA about 5 pounds. My filter is new as of a year ago.



Postby Guest » Mon 23 Jul, 2007 16:02

make sure you've scrubbed all surfaces to get rid of the algae spores, we had this same problem with green algae, pool would clear but not hold chlorine, then go cloudy/green again went on for 2+ months. Then we discovered the algae growing inside our drop in steps, removed and cleaned them 10+ days ago and pools been back to normal ever since.
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Postby chem geek » Mon 23 Jul, 2007 16:29

Do not add any more CYA to the water. In fact, you should not be using stabilized chlorine (i.e. Trichlor pucks/tabs) since that increases the CYA level. And you should not use powdered shock that is Dichlor since that raises the CYA even more. Instead, you need to add unscented bleach or chlorinating liquid to your pool to raise the Free Chlorine (FC) level to 20 ppm and keep it there by testing several times and adding more chlorine to keep the level up. If you do not keep sufficient chlorine level, the algae will grow faster than the chlorine can kill it -- initially, the chlorine usage will be high as it kills and oxidizes the algae.

See this link for how even a pool with lots of algae on spring opening is cleared with chlorine alone. You do not need any other products except chlorine unless you have an above ground pool with no floor drain in which case you may need a flocculant (OMNI Liquid Floc Plus), but only after you've initially killed the algae with chlorine and keep the chlorine level high. You can vacuum to waste dead algae that settles to the bottom of the pool and will also need to clean (backwash if sand) your filter.

You need to keep the FC at 20 ppm until the following 3 things happen: 1) the water gets crystal clear, 2) you measure minimal FC drop overnight (< .5 ppm; certainly less than 1 ppm), 3) you measure minimal Combined Chlorine (CC) of <= 0.5 ppm. A pool can be clear and yet still be consuming chlorine due to algae. You can most readily tell this by whether the chlorine holds OVERNIGHT. During the day, chlorine will get reduced by direct sunlight, though usually you don't lose more than half the Free Chlorine level in a day. It's the overnight loss that tells you if you have algae or other organics in your pool that need to be shocked to get rid of.

You need to get yourself a good test kit, the Taylor K-2006 (usually online since most pool stores do not have it -- the K-2005 is NOT the same) or the even better TF-100 kit from tftestkits(dot)com. After your algae is completely gone and your chlorine usage is normal, you still need to maintain a Free Chlorine target of around 5.8 ppm FC with an absolute minimum of 3.8 ppm FC.

If you continue to use Trichlor pucks/tabs in feeders, then I suggest that after you have done the shock, you use a weekly maintenance dose of PolyQuat 60 algaecide. Otherwise, you will have to increase your FC level as the CYA level rises (target FC of 11.5% of CYA and minimum FC of 7.5% of CYA) and it will be harder to keep up. With the algaecide, you can have a lower FC level even at higher CYA levels. Another alternative is to switch to using bleach or chlorinating liquid instead of Trichlor, but you'll have to add chlorine probably every day if you do that unless you get The Liquidator for automatically adding bleach or chlorinating liquid to your pool.


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