Help - Green Pool


LifeTime Supporter
May 17, 2009
So the pool turned green last week. This is only the second time in the fours years I have had it but I can't seem to kill the algae.

I have been using PACE - Clear Shock 99% sodium dichloro-s-triazinetrione hydrated to shock the pool.

I added a couple of bags of shock last Friday and ran the filter for a day and no change. Still green. I got out the test kit and started monitoring the pool for the last couple of days while treating. Here are the results over the weekend:

ph - 7.4
TA - 80
FC - 25-28
CC - 0
CYA - Assumed around 50, did not measure

So after another two days of adding shock to keep FC close to 30 I finally measured CYA and it looks like CYA is around 80-90.

Here are last nights numbers

ph - 7.4
TA - 80
FC - 26
CC - 0
CYA - 80-90

I read the CYA/shock chart in pool school and it looks like for my CYA level and it looks like I my FC never reached shock level (close to 40). I added another bag this morning and the FC is 48 and CC - 0. I'm hoping to see some progress when I return from work tonight.

Here are my questions:

First any suggestions on anything else I should do?

Is my shock product adding CYA?

At what chlorine level should we go swimming again?

If I lower FC back 3-5 will my pool still be sanitized with the high CYA levels?

Once I kill the algae should I lower the FC with sodium Thiosulfate back to 3-5 ppm?

Should I drain/refill to lower CYA or will it go down over time?


TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
PACE - Clear Shock contains what we call dichlor. Dichlor is constantly adding CYA. As the CYA level goes up, you need to aim for a higher FC level, which means more shock, which raises the CYA level again. It is a vicious circle which is impossible to win. You need to stop using dichlor. At this point you probably also need to replace some water to get the CYA level down before you will be able to get control of the algae.


Mod Squad
TFP Expert
LifeTime Supporter
Jul 10, 2009
Houston, Texas
As long as your CYA remains 80-90, your daily FC should be between 6-11 according to the pool calculator. Keeping the FC between 3-5 will not keep your water sanitized until you drain and refill to lower the CYA.


LifeTime Supporter
Jun 4, 2007
If you are using the Aqualogic SWG you have to use something else for shocking, and your CYA is on the high end already.
Shock with bleach as it doesn't add anything else to the water that you don't want in the water (OK, maybe a 'little' salt).

Drain ~25% of the water, shock the heck out of it with bleach & brush & brush.. Keep checking your pressures to know when to clean filters. Don't even bother with the SWG until the algae is gone...

When was the last time that your SWG cell was cleaned?

Terry in NC :cool:


LifeTime Supporter
Jul 29, 2009
I'm going to offer a slightly different opinion.

With a SWG, I do not think that 80 or even 90 CYA is out of bounds. And if you are in a lot of direct sunlight to boot, that may even be where you would want to be. If you can keep your FC at about 6, you should be OK. But if nothing else, you definately do not want it to rise further, so stay away from dichlor products. (Bleach is great).

And, your shock target of 30 really should be getting the job done.... And now 48.

A couple of things that concern me:

1) Even with a CYA of 80, 3-5ppm steady chlorine through a SWG would make formation of algae unusual. Not unheard of... just unusual.


2) That algae surviving days of 30ppm... also unusual. If you were clearing a swamp that had no chlorine input for months and months, I would be surprised not to see SOME color change after a few hours at those levels.

If it were me, I'd turn off the SWG and do an over night FC loss test. If you are not loosing FC overnight, you might be looking at something other than algae.

Otherwise, I would want to make sure that the SWG was working properly, and in the future do at least weekly FC tests.


LifeTime Supporter
May 17, 2009
Problem Resolved!!

First thanks for all the helpful responses. I know why it turned green. I got a little complacent and did not check the FC for over a week (vs my normal 2-3 times a week) combined with some rain followed by hot days.

My SWG was clean and running the whole time. The biggest issue was the PACE (dichlor) shock. It was raising the CYA with each bag I added, thereby reducing the effectiveness of the chlorine, so I was never getting to shock levels.

I drained some water and refilled. Throwing a couple of bags cal-hypo shock and running filter for two days did the trick. I am now lowering the FC level using ChemOut to get the water back to a safe wimming level.

I typical only hav to sock the pool about thre times in the summer. I am able to reguate the FC sing the SWG and rarely have a mesureable CC level.

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