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Good morning, I have a customer's pool and that I'm having a hard time keeping chlorine. Here's the current details.
36000, gunite w attached spa.
TH 225, TC 3, FC 3, pH 7.6, TA 100, cya 90
Phosphates at least 1750 & minimal to no nitrates. Also, they have a Jandy (4 cartridge filter), and a 60000 gal aquapure cell which is at 80%.
Last Wednesday I received a call that there was algae. Thursday, 7.5 gal liquid chlorine, brushed pool. Friday, vacuumed, cleaned filter & 0 FC reading so I added another 7.5 gallons chlorine. Monday FC 5, Tuesday FC 3. Going back today. The cell is functioning correctly and the salt levels are 3560.500 ppb phosphates in supply water. Friends company says I should add 30 gal of chlorine and then everything should be fine and once that chlorine is used the cell will be able to maintain proper chlorine levels again. I think I should get those phosphates down to below 300, preferably 100 which will be quite tedium given the filter type and the fill water. In past, I have removed phosphates down to 100 or less and the cell took over and was able to maintain the chlorine levels. Not sure what to do. I don't understand the reasoning behind my friends business approach for the system. I think that once all that chlorine is gone we will be back to square 1. Please help. Any and all info or methods will be greatly appreciated! Thank you, Joe

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Check your TDS level. High TDS can screw things up.

TDS is going to be through the roof. Pool has a salt chlorine generator.
You have to let your clients know .. ( this is important) a pool needs to be drained once every 3 years . 5 years max.
Due to the cities hard water with all the minerals and phosphates that go in their NO MATTER WHAT. ( I KNOW IN CAL. We are in a drought so, this isn't an option.. BUT.. ITS NOT A PROBLEM TO KEEP BACKWASHING AND RECHARGING W/ DE EVERY WEEK YOU GO. Ha ha). You do what you have to do .. SO US POOL GUYS DONT GET CRAP ALL THE TIME.. having a "thankless job".
But you CYA IS HIGH IF YOUR USING CHLORINE. IT SHOULD BE ( 30-50PPM)
IF SALT YOU CAN RUN CYA HIGHER THE MAX ( 80PPM).
BUT YOU SOUND LIKE YOU HAVE CHLORINE LOCK. IF YOU USE LIQUID CHLORINE, YOUR ONLY PUTTING IN COMMERCIALLY 12.5%.
USE CAL HYPO "MAXIMUM STRENGTH WHICH IS 73%.
( no cya in cal hypo. And it hold better than liquid.
( AND CHARGE YOUR CLIENT A "ALGAECIDE TREATMENT "FEE @ $ 6.00 a lb" .. To earn your money back
( both liquid and cal hypo are considered... " NON BALANCED" ( I am blanking on the correct terminology)..
BUT BOTH DO NOT HAVE "CYA" but liquid gets burned up so fast.. NOT FROM THE SUN BUT FROM THE "UV RAYS FROM THE SUN".. If you add your liquid in the day.. "STUDIES AND ARTICLES SAY IT LASTS ABOUT 21/2 hrs IN YOUR POOL BEFORE "GETTING USED UP"
WHEN I USE CAL HYPO- IT STAYS IN MY POOLS
GOOD LUCK

The answer is simple in this case.  Your Free Chlorine (FC) level is too low for your Cyanuric Acid (CYA) level.  The amount of active chlorine (hypochlorous acid) that kills algae (and disinfects) is proportional to the FC/CYA ratio.  In order to prevent algae growth independent of algae nutrient (phosphate, nitrate) level, you need to maintain a minimum FC/CYA ratio of 7.5% in a manually dosed pool.  For a saltwater chlorine generator pool (such as what you describe) the minimum FC/CYA ratio is 5%.

If you don't want to dilute the water to lower your CYA level or to raise the FC level to around 4.5, then you can lower your phosphate level with a phosphate remover.  That should let you operate at a lower FC/CYA ratio of around 3% so 2.7 ppm FC with 90 ppm CYA.

It is true that you no matter your longer term plan you need to elevate the chlorine level to get rid of the existing algae and using chlorinating liquid or bleach is the best way to do that without increasing the CYA level (from Trichlor or dichlor) or CH (from Cal-Hypo).

You can learn more about pool water chemistry in the Water Chemistry section of this forum or in the Pool School at Trouble Free Pool.

Thanks for the replies! I did get everything under control. I killed and cleaned out the algae. Got the phosphates down to 600 as of Saturday from over 1800ppb. Little bit more and the cell will be able to take over on its own. I had the cya up a little higher w the salt, so you were right about just needing a little more FC. Thanks all!

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