Chlorine gone overnight - what is happening?

[septemberbrain]

We have an in-ground 25,000 gallon gunite pool that uses a SWG. I have been battling mustard algae for the last month. I have used a number of different algaecides which hardly worked. Finally got a copper based algaecide which worked and also shocked the pool every few days, sometimes with bleach, usually with dichlor shock.

This week I have been brushing the pool on a daily basis to make sure algae does not return. Two days ago I put in 3 lbs. of dichlor shock. Yesterday I tested my water with two different test kits. FC was above 3. Took two bottles of water to two different pool stores and got different numbers from each, which adds more confusion, but still FC was above 3 in both those tests. Today I tested the water again and FC was below 1.

What the heck is going on? I do not see the algae growing again but have maintained my brushing routine. I am at a loss. I'm almost ready to dump all the water from the pool and refill it. Can anyone help me with this? Am I still battling the algae or is there something else that is using up all that FC? Could it be the algaecides?

I've tested the water coming out of the salt cell and it has FC so I know that it is working.

Thanks in advanced for any help.

 

[JohnT]

What is your CYA level?

 

[linen]

Welcome to tfp, septemberbrain

As JohnT said, let us know what your cya level is. What are you using for a test kit? Pool store results are notoriously unreliable. Also, if yours or theirs are from test strips, don't believe them, test strip results are often wrong. See this link for appropriate drop based test kits: http://www.troublefreepool.com/pool-school/pool_test_kit_comparison

 

[septemberbrain]

CYA levels are either 60 or 100 depending on which store results you want to believe. I know that the second store uses a Taylor test kit, their results were 100. So I am fairly confident that I have enough CYA in pool. Would too much CYA cause FC to drop so quickly?

 

[duraleigh]

Chlorine is consumed by two things.....sunlight and organics in the pool. Since you experienced the loss at night, you have organics (algae) in your pool and will need to start the shock process to get rid of it.

read "The ABC's of Pool Water Chemistry" up in Pool School then, read "How to Shock Your Pool".

 

[fast1971chevelle]

Have you done a oclt or tested for cc? As stated above pool store results are way wrong MOST of the time so I would not put to much faith in their results, you really need to order your own recommended test kit and the only place to get it is from tftestkits.net and the tf100 is the best bang for the buck and super fast shipping......Mike

 

[septemberbrain]

The first store I went to was Leslie's. Wouldn't they use their own test kit?
BTW - my goals for today:
1) completely brush and clean pool - I rented a dive tank yesterday so I will spend a hours under water doing this.
2) Clean the pool filter which is a cartridge filter. I did this last month but will do it again this morning.
3) Shock with bleach every day for one week.

Thanks for all the responses, Wish me luck!

 

[Vinoveritas]

If your CYA is 100 or higher (and it may be higher with all the dichlor) you are going to have a very hard time shocking the pool and if CYA is much over 100 you may want to consider a partial drain. Read pool school and get a good test kit and best of luck!

 

[linen]

I would echo what Vinoveritas said, if you are fighting algae, a partial drain/refill is needed to lower your cya level (if you trust the results). For swg's, we recommend 70-80 ppm cya. Even that level is high if you have to do the shocking process.

To do the shocking process, you will be most effective with your own appropriate test kit as mentioned previously.

3) Shock with bleach every day for one week.

Your approach to shocking may or may not fix your issue, but for sure it is a blind way of doing a shock process. See the link above for our recommend approach.

 

[anonapersona]

Please, please, get a good test kit before you spend a lot of money on the right things in the wrong amounts.

Around here, we stress that you need to test to see what your pool needs, then calculate exactly how much of what chemical you need, and add that, only that, to the pool. Do not use combination chemicals or magical potions that promise to fix a multitude of problems. Then you look to see if you got the expected results, test again if necessary, and keep at it. This is what solves problems.

If you have high CYA then quite a lot of the chlorine you add to the pool is not available to kill algae unless you are at very high levels. Most test kits are not reliable at high levels. Adding a random amount of bleach to the pool on a timed schedule will not assure you that you have reached the proper concentration. Nor will it tell you when it is used up and you need to add more bleach. Repeated testing, at home, when you want it done, is the answer.