We are experiencing a strange problem and would appreciate it if somebody could offer any suggestions. This is our 6th year with a 24' round above ground pool and never had this problem before. It's like our pool is eating the chlorine as fast as we put it in. Upon startup we've shocked the pool at least 5 times now but the total and free chlorine levels are not budging must past 0. Water started out cloudy but with the application of algaecide, is now crystal clear and it's been that way for a couple of weeks. The last 2 shock attempts we've added 2 gallons of 12% liquid chlorine shock, and even that did not budge the levels. We are also running Chlorine tabs through a chlorinator. We've balanced out all other primary chemical readings which I will list below. The one thing I heard that could be causing this is a high phosphate level. We have not tested for that yet. But the pool location has not changed in 6 years and not sure why all of a sudden it would show a high phosphate level this year. We are in Massachusetts. Openings in previous years only took 1 shock treatment with 1 gallon of 12% liquid chlorine and we were fine after that. I am leary to just keep upping the chlorine shock levels because it's a waste and it hasn't worked yet and we are up to 2 gallons, my next step would be to dump in 3 or 4 gallons but I don't want to do that without exploring other options first. Perplexing to say the least.

Any suggestions would be appreciated.

(note: readings from 2 diff. water test systems offered via retail store; also have supplemented with latest readings via test strips)

Free Cholorine 0
Total Chlorine .4
Combine Chlorine .4
Total Alkalinity 120 ppm
Ph level 7.1
Calcium Harness 130 (currently adding in Calcium to bring up)
Cyanuric Acid 40
Copper 0
Iron 0
Total Dissolved Solids 0

This has nothing at all to do with phosphates. Most likely it is ammonia in the water. Ammonia reacts very very quickly with chlorine. There is nothing to do but keep adding chlorine.

You should not add huge amounts of chlorine all at once. The best thing to do is to add enough chlorine to reach shock level, about 15 to 20, over and over as often as once every hour, until you get a measurable FC level after an hour.

Thanks for suggestion. Should we try to test for Ammonia levels? We are adding cyanuric acid which hasn't finished dissolving yet. Would the flattening or decrease in cyanuric acid levels confirm that it's being converted to ammonia?

You can do an ammonia test if you have an appropriate test kit, they make ammonia tests for aquariums. Doing so isn't required. The solution is the same regardless. If you lost CYA over the winter, then the CYA almost certainly converted into ammonia. However, ammonia can subsequently off gas or be broken down over the winter, so it doesn't automatically follow that you have ammonia when CYA has vanished.

Ok Thanks. Regarding testing every hour. Will the chlorine have a chance to fully circulate in that time?

Yes, an hour is definitely enough time. Just keep the pump running.

Sorry for continued questions, but I just want to make sure I have this right. When you say shock it to bring it up to 15-20 ppm, do you mean free chlorine? Your calculator tells me I would need 2 gallons of 10% chlorine to achieve free chlorine of 15 ppm (13,500 gallon pool). Kmart has a sale on 10% chlorine - 1 gal. for 3.99. I may go down there and pickup 10 or so. Do you find that these situations could take 5+ shock cycles to over come? Is it a must to re-shock within a short time frame, or can they be spread out over a few days?

Thanks again for your help.

Every situation is different so we can't say how long it'll take. Shocking isn't a cycle it's a process. Just because you need to add 2 gals this time you may not need 2 gals next time. You'll only know how much to add through testing. The object is to get it to shock level quickly and keep it there. Since that's not completely practical our recommendation is to add what the Pool Calc says then test and add in an hour and continue until you're done shocking.

Welcome to TFP!

Shocking a pool is a process not a product or 1 time application. Just like Jason said you need to bring your FC up the shock level about 15ppm and keep it there until the FC holds. That it why you need to add chlorine very often. You are losing chlorine because what ever is in your pool it is eating it faster than you are putting it in. I suggest you read this topic in pool school How to Shock a Pool.

That's an excellent question and often the most misunderstood portion of what we teach.

Our definition of shocking a pool is a process that can take place over several days but it is important there is no interruption.

So, simply the high dose of chlorine is not shocking the pool. Shocking the pool is the PROCESS of bringing your FC suddenly to very high values and then HOLDING THAT VALUE CONSTANTLY until your water is crystal clear.

In order to do that you must test and add very often. If your shock value is 20, dose enough FC for 20. perhaps an hour later, you test again and the FC is 8....you add enough FC to return to 20pp. You continue that process of always replenishing to 20ppm until the pool is crystal clear.

Brushing, vacuuming, cleaning your filter and running the pump 24/7 are also integral parts of this process.

It can take overnight or perhaps a week (rarely more) but the key to understand is that it is a process of multiple applications of chlorine.

In this particular case, where the water is clear and ammonia is suspected, FC will very likely go down to very close to zero after an hour several times in a row. It also isn't crucial to add more chlorine every hour. Adding chlorine as infrequently as once a day will be fine. If it was algae, twice a day is the minimum and more frequently is important at the start, but with ammonia once a day is sufficient, though more often will get the situation resolved more quickly.

Yes, 5 cycles is a good guess, though really there is no telling. If you buy too much, you can always use it later.

Thank you all for your input. To clarify, as mentioned in my initial posts I have been going through the process of shocking for 3 weeks, and have probably shocked it 6+ times at 1 gallon and more recently 2 gallons each time. Algaecide was added at the beginning and the water has not been cloudy for over 2 weeks and really is crystal clear. This is why it has become perplexing for me especially since the previous 6 years of pool openings only required a 1-time shock with 1 gallon of chlorine and it was good to go.

Thanks to Jason for identifying ammonia as the potential culprit. At least I can stop scratching my head for a while and target this as the problem. What Jason says about cyunaric acid gassing off in the winter causing ammonia seems to make sense. Interestingly enough, last summer was the first time that I actually made a specific effort to bring up the cyanuric acid levels by adding it in (tri-chlor pucks seemed sufficient before, but we had really hot sunny weather). This is consistent with it being the first time I'm seeing this ammonia problem with this season's opening.

Also, I've called 2 local retail stores that do pool water testing and neither of them knew anything about the potential problem of ammonia eating chlorine, or cyuanaric acid gassing off. And their testing doesn't look for ammonia.

I had this problem when I opened this year, also. The water was sparkling clear but just would NOT hold chlorine. I had to order cya reagent as mine was gone. Well, there was my problem, cya was reading zero after the brutally cold winter. I too wasn't sure what was happening until I got on the site and started reading about cya converting to ammonia. So, I just followed the advice of keeping it at shock level as much as I could, adding more cya to get to the appropriate levels and viola!, after a couple of days it started holding and I haven't had a problem since. One thing I can vouch for, is the mods and admins on here know their stuff!