Chlorine Block


New member
Jun 23, 2010
OK I've had it with this pool already and its not even July yet. I appear to be having a problem with the mythical chlorine block/lock no matter how much shock I add within hours I always have a 0 reading for FAC and TAC.

We started out the year just fine and then just about 2 weeks ago started to have a problem with mustard/yellow algea. We purchased a product called Yellow Out and followed the directions given to us which was to add the Yellow Out wait 5 minutes and 3 bags of shock, 12 hours later add 3 more bags, 12 hours later add 3 more bags. So after all that the pool water is clear but chlorine levels are @ 0. I added 3 more bags Still @ 0, 3 more still @ 0.

Several days after adding the Yellow Out I had a yellow frothy substance that was floating around in the deep end of the pool. I'm assuming this is the dead yellow algea because before it wasn't floating it was on the bottom of the pool. Anyway I skimmed just about all of it out and as I see new pockets develop I'm skimming them out.

Yesterday a took a sample to the pool store they tested the water and said I had a Chlorine Block and that I needed to add 6# of shock. I added 8 to be on the safe side. I added it last night @ 10PM no sun to help burn it off, this morning before sunrise I tested it and it shows 0.

Here are the results from the pool store yesterday

PH 7.2
TA 80
Hardness 130
CYA 40

This is an inground 18x36 pool aprox 25,000 gals salt water pool.

So in the last week I've added aprox 23 pounds of shock, while the whole time my chlorinator has been on super chlorinate, and I still show 0 FAC and 0 TAC. Personally I'm clueless as to what to do, and quite frankly I don't think the pool store people are any better.

Any advise would be greatly appreciated.


Mod Squad
TFP Expert
Apr 4, 2007
SW Indiana
Welcome to TFP.

You need to keep adding chlorine. With algae or any kind of contaminantion, you can use up all your chlorine in a few hours. We usually recommend checking it every hour for the first 4 or 5 hours, then every 4 hours the next day or so, then every 8 hours until the water clears.

Here are a couple of links to get you started:

Defeating Algae

Turning Your Green Swamp Back into a Sparkling Oasis

Shocking Your Pool


Well-known member
Apr 26, 2009
Northeast Ohio
Welcome to TFP.

I'm not sure why you're consuming so much chlorine (someone else will likely know, but I suspect the yellow out.) What "shock" product have you been adding to the pool? I'm concerned that the pool store readings are not accurate, especially the CYA level. I'm also concerned that with everything they're having you add, your chemistry is soon going to be so out of whack that they tell you do drain a couple of times and then everything gets started over again.

If you have mustard algae, you really need to get your pool to mustard algae shock level and hold it there. To shock, I'd immediately switch to liquid chlorine or unscented household bleach. They're the same chemical; however bleach is usually 6% while liquid shock is often 10% or 12.5%.

You also need to get a good test kit of your own. To get a handle on what is happening in your pool, you're going to need to be able to test more often than the pool store can do and more accurately. I recommend the Taylor K-2006 or TF-100 (the TF-100 is a better overall value and ships very fast.) The Taylor K-2006 is generally found only online (do not get a K-2005, it's not the same kit.)

At this point, you should turn off your SWCG and switch to liquid chlorine to save cell life. How deep is your pool? If you already know how many gallons it holds, you can go to and put the gallons in the top, 0 for NOW in FC and 24 in Target (24 is mustard algae shock level for CYA of 40). You should test hourly and put that number in "now" and leave 24 in Target and then add that amount of bleach.

Also, you're going to want to spend as much time as you can reading Pool School. There is a lot to absorb going from the pool store telling you what to do to really understanding it yourself. Make sure you post any question you have.

I really can't stress enough how important the test kit is. You're going to get this problem cleared up much faster if you have accurate test results.

Bama Rambler

Mod Squad
TFP Expert
Bronze Supporter
LifeTime Supporter
Jun 22, 2009
SouthWest Alabama
What are the ingredients of the "shock" you're using? I suspect that either the hardness number or the CYA number OR both are wrong!
From your post you've added 23lbs of shock. If that's correct and the shock is Dichlor you've added 56ppm CYA or if it's cal-hypo you've added 50ppm calcium.

[edit]I had a whole bunch more typed but Loughps beat me to it so I'll let their post stand for me too.[end edit]


Platinum Supporter
LifeTime Supporter
Apr 14, 2010
Stephens City, VA
You have come to the right place and you are in good hands. I absolutely agree with the test kit. You will not be sorry. The pool places are notoriously inconsistent and just plain wrong. They only want to sell products that you don't need or can get elsewhere and will likely screw up your pool even worse. Ours uses strips put into a little machine and a computer printout spits out what is needed. As an example of what I'm talking about, I took my water in to the pool place to get a salt test yesterday afternoon before adding salt to my pool and of course they ran the whole panel...after looking at me like deer in the headlights when I specifically asked for the salt test.

Their results: (my results in parentheses)
FC: 3.3 (5.5)
CC: 1.9 (0.0 )
TC: 5.2 (5.5)
ph: 6.8 (7.6)
TA: 190 (100)
CH: 425 (didn't test)
CYA: 30 (45)
TDS: 110 (told me this was the salt number)
Phosphates: 100 (bogus thing to test)

My shopping list there was:
1.75 lbs of PH up (can use borax instead to raise PH, but I'm aerating right now to cool water, which will raise ph so adding ph plus would have sent my ph through the roof!)
15 lbs ph down ( to lower TA, muriatic acid does the same thing)
3 qts metal magic (not needed for CH in a vinyl pool so that was literally throwing $$ down the drain)
4 gallons of liquid shock or 4lbs power magic (to raise to shock level a pool that has no CCs and is crystal clear, and incidently is calcium hypo which would raise my 'dangerously high' calcium level).

That would have cost me around $100.00 or more. I'm not sure. Yesterday alone my own test kit paid for itself at least 1.5 times and I saved myself from screwing up my pool. The above example shows just how important it is to know WHAT you are putting in your pool, WHY it is going in there, HOW to add it, and the combined effects of doing so.

You are in good hands. Stay with us, update frequently and you will love your pool again.


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
Hi, Jimmy,

Welcome to the forum. :lol:

loughps has added a good clue to your dilemma. Yellow Out sucks up chlorine like the intake on an F-16 sucks up air!!.

One of the prime things to learn on the forum is never put things in your pool you don't need.......which is almost everything in the pool store!! :shock: :shock:

Good advice above. Everyone here will help you get the pool manageable and crystal clear but you'll have to trade in that bad habit of spending money at the pool store. :lol: :lol:


LifeTime Supporter
Nov 18, 2009
Sacramento, CA
PG22, I love that they wanted to sell you pH up and pH down at the same time!! What did they think the effect of 1.75 lbs of PH up would be after adding 15 lbs ph down?


Platinum Supporter
LifeTime Supporter
Apr 14, 2010
Stephens City, VA
I dunno..but the bioguard dealer had totally different results still and the shopping list there....well, let's just say it would have made my car payment this month. So, again, OP you need your own test kit.


New member
Jun 23, 2010
Thanks to all who've replied, I really appreciate it. 4 years of owning this house and never a problem with the pool except for this year. This yellow/mustard algea threw me for a loop.

Loughps - I do already have the k-2005 kit, what the main diff between that and the 2006 and should I purchase the K-2006 if I already have the K2005? Also thanks for the advise and pointing me to the pool calculator.

Bama - Yes the shock I've added was Dichlor and now my CYA has jumped from 40 to 100. Whats the fix for that? will is just burn off on its own or do I need to add something to lower it?



TFP Expert
LifeTime Supporter
In The Industry
May 20, 2007
SWSuburban Chicago, IL
You'll have to replace water.
No more Dichlor, liquid chlorine only
The difference between the 2005 and 2006 is the FAS-DPD chlorine test, and a must have now in your situation. You can order it ala carte from TAylor and for about $25 but it's essential.

Now, your CYA level for an SWG pool - I'd target 60. You'll need to replace about 1/3 of your water, recirculate, retest and repeat to see where you are at cause it's very possible that you are much higher than 100.

Can you put your pool's specs in your sig? Go to User Control Panel, then Profile, then Edit Signature.


LifeTime Supporter
Jun 3, 2008
When we purchased our house the pool came with mustard algae. We poured chemicals into the pool and brushed daily. We made no progress for over one month. Mustard is very very difficult to get rid of. It takes a very high shock level to kill mustard algae. If a little mustard algae is left then the algae continues to grow even though you are adding more chemicals. You can play this game forever!

We added copper algaecide to get rid of the mustard algae. The problem with copper algaecide is that it can and probably will stain your pool. The best solution is to raise you Cl level very high; however if your CYA is really 100 the shock level must be extremely high. Pool store tests for CYA are generally wrong! You need to conduct your own tests and believe in your test results.

You have followed enough of the pool store advise to know that you need to do something different. Try our advise for two weeks.

My recommendations:
1.Test your CYA with a view tube test. If your CYA is 100 you will need to replace water. If the 100 was from a pool store test then 100 is wrong.
2.You need a FAS-DPD Cl test. You need this test to accurately measure the FC level. You need this test to conduct an overnight test to determine if you have killed all the mustard algae. Currently you are operating in the dark and do not know the status of your pool.
3.You should shock your pool with 6% bleach. Use the CYA/Cl table on this site to determine the shock level you must maintain until the Cl level holds overnight.

Your “I've got this licked moment” will come when you shock the pool with bleach and the overnight test shows less than .5 ppm FC loss. You may need to run your filter for a week to clear out the dead algae. When the FC level holds you know that you have killed the mustard algae & you just need to filter. Beware that mustard algae can be re-introduced into your pool from pool floats, swim suits, etc.

Be prepared to go to Walmart or SAMs to purchase bleach by the basket full. This will work. This will take some patience. We have all tried pool store solutions, decided to test our pool water at home, and we have control of our pools at low cost. Give this a try.


Well-known member
Jun 2, 2010
Fort Worth, TX
Do you have a SWG? because if you NOT trust the reading on the SWG panel! Trust me, get an independent salt test kit such as the Aqua Check strips or the K-1766 test kit.


LifeTime Supporter
Jun 18, 2010
As a new pool owner myself and also an owner of the K2005 you need to get the TF100. The K2005 did not show me the correct chlorine levels and I wasted a lot of money on chlorine trying to fix my problem while using it. I received the TF100 a few days again and I saw immediate results the following morning because of the accurate readings above 5ppm. It went from green to blue overnight once I could see the results thanks to the TF100. I'm still working on my pool but I'm close to the end. Good luck and don't give up. I almost did and everyone here has helped me soooo much!! 3 days later I am very happy with my pool. I hope everything goes well for you.


Well-known member
Dec 3, 2009
Central Massachusetts
sonflower said:
Do you have a SWG? because if you NOT trust the reading on the SWG panel! Trust me, get an independent salt test kit such as the Aqua Check strips or the K-1766 test kit.
I'm going to put a couple of caveats on this advice and contradict it a bit. First, the Aquacheck strips are not all that accurate. Most reports seen at TFP indicate that the strips run high. I have seen them run as high as 1000 ppm over the SWG panel as well as electronic salt meters. Second, the reading on the panel is what the SWG "goes by" in terms of the proper function of the unit. IOW, if the panel reads 2300, and it wont run until the salt reading hits 2500, then add salt. Even if the drop kit, salt meter, etc, say it's 2800, it doesnt matter. Add the salt needed to get the panel to read 2500. I will say that the drop kit or salinity meter are good to keep you in the ballpark. If the panel reads 1200, and you know there is 2800 in the pool, then the panel needs attention. But if your talking about 500 to even a 1000 ppm swing, go with the panel.