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I have a 15,500 gallon inground plaster pool. I recently purchased the taylor 2006 kit. Doing the tests, this is what i got.

FC 0 (the reason for the 0 chlorine is, we recently purchased the house and prev owners didn't seem to have added any chlorine during the 3 weeks period. Only trichlor jumbo tabs were used as i could tell from the floater and trichlor bucket that we left behind.)
CC 0
PH 7.5
Total Alk 180
CH (using the taylor 2006 kit, at 10ml, it took 22 drops). So 22x25 = 550
CYA (it's way below the 100 mark on the test tube) It's on the 1 1/2ml mark on back of the tube which i would say more like 300 cya. I can't tell exact measurement since tube is not labeled below the 100 mark. 

I added half gallon (10% liquid chlorine) but i still can't read any chlorine on the kit. I'm guessing it's because of high CYA. Water look clear. No signs of cloudiness or algea. What do you suggest i do at this point. How come the water still looks good even though no chlorine in water and high CH. What do i adjust first at this point and how do i get free chlorine up to it's proper level 3-4ppm.

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there is some thing buffering the chlorine. Let it run a day or two and retest- You might need to backwash - also the fact that the water is clean is clean. Just make sure the PH is desirable otherwise it could cause bad red eyes when you swim in it. –  ppumkin Aug 5 '11 at 8:46
    
Just a tip. Put about 1/2 liter in a bottle with a cap and take it down to the nearest pool supply store. They will do a complete analysis for free in hopes of selling you whatever chemicals you need to fix the problems they uncover. –  JohnFx Aug 5 '11 at 19:29
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2 Answers

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CYA can be higher than 20-40 without any problems. I'd say it only become a problem when you start going over 100. It's true that you can't get rid of it without partially draining your pool, though.

Your CYA should be ~40ppm, so once you figure out what it is, adjust it as necessary, partially draining and refilling the pool to lower it, or adding CYA to increase it. (I haven't used a taylor test kit in probably 10 years :S Are you sure it's really 300 ppm? If so, I'd pretty much drain the entire pool... lol.) CYA keeps the chlorine from evaporating off and is very important, but having too much CYA makes the chlorine ineffective. Once you get your pool to ~40 ppm CYA, add bleach at night (to help prevent evaporation) with the pump running.

If you find that you are still unable to hold FC even with a good amount of CYA, then you have "chlorine lock," which is a situation where, when you add chlorine, it combines with something (typically ammonia or urea, created typically by not maintaining proper chemistry, at which point your chlorine evaporates, followed by microorganisms eating your CYA, turning it into ammonia and urea) very quickly. Some people out there kick and scream when they hear the words "chlorine lock," even though they do not question that there are situations in which free chlorine will be gobbled up very quickly. They hate the term because they hate the pool chemical industry / pool maintenance companies, but pretty much everyone agrees on the solution, and it's very simple: you need to add a large amount of chlorine into your pool, whether it be chlorine shock (that does not contain CYA) or liquid chlorine bleach. When you have chlorine lock (i.e. an ammonia/urea problem,) adding a half-gallon of bleach is like attacking a ten-acre fire with a garden hose. It's the right idea, but not on the correct scale.

Here's how to fix the problem:

  1. Dilute a small amount of your 10% chlorine bleach -- three parts water, one part bleach. For example, 1 fl oz bleach, 3 fl oz water.
  2. Get three one-gallon buckets and put one teaspoon of the diluted mixture into the first bucket, two teaspoons into the second bucket, and three teaspoons into the third bucket.
  3. Add a gallon of pool water to each bucket, stir gently, and put the buckets in the shade for 4 hours.
  4. Test the chlorine levels of the buckets after 4 hours. Whichever one has 2-3 ppm of chlorine left, is the one you want to do your math on. If none of them have 2-3 ppm left, add 3 teaspoons to each bucket, stir gently, put the buckets in the shade for 4 hours, and test again -- repeat the process until you reach 2-3 ppm.
  5. Take however many teaspoons you added into the bucket that read 2-3 ppm, and multiply that number (1, 2, 3, etc.) by 4.5 (math below), giving you how many gallons you should add into your pool to fix the problem. Add the bleach at night, with the pool pump running.

1 diluted tsp / 1 gal pool water = 15,500 diluted tsp / 15,500 gal pool water

15,500 diluted tsp / 768 tsp/gal = 20 diluted gal

20 diluted gal = 5 full-strength gal

Use 4.5 instead of 5 to be on the safe side. This likely won't be quite enough, so if you find that your pool is still unable to hold any FC, add 1/4th of a gallon of bleach each night until it starts holding some FC.

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Thanks @Michael, Are you suggesting that if i shock the pool with high level of 10% chlorine(no cya), then i can lower cya or balance it. I live in California and water here is expensive. I dumped half a gallon of chlorine because the calculator at poolcalculator.com suggested i add 58oz of 10% chlorine to get FC from 0 to 3ppm based on 15,500 gallons pool. –  Hussein Aug 6 '11 at 11:39
    
CYA decreases the effectiveness of chlorine, but it also decreases its harshness (on your eyes, for example) and helps keep it in the pool. To some extent, yes, if your CYA is a little high, you can simply add more chlorine. If your CYA is at 300, though, then that's far too high, and you're going to have to partially drain it. If you are 100% sure it's at 300 ppm, then you need to drain ~80% of the water. There are methods of dealing with CYA, but none that are economical. If water is expensive in your area, you might want to look into rain water harvesting. –  Michael Aug 6 '11 at 13:03
    
Out of curiosity, what type of chlorine are you using? Dichlor and Trichlor tabs add significant amounts of CYA -- almost as much CYA as chlorine. After one year, you can easily add 100-200 ppm CYA if you're using them, requiring you to drain the pool. I'd recommend using either liquid bleach or getting a salt water generator. –  Michael Aug 6 '11 at 13:05
    
I'm using this chlorine liquid lowes.com/pd_77608-1327-331_?PL=1&productId=1195991. You said half gallon chlorine is not enough for 15,500 gallon pool as. This is what poolcalculator.com gave me to raise fc from 0 to 3ppm. As i mentioned in my question the only chlorine used for the past 3 weeks was stabilized trichlor tabs. I don't know how long they have been used in the pool. I just recently added half gallon 10% chlorine. Water looks sparkling clear. I wonder why no algea. –  Hussein Aug 6 '11 at 23:40
    
You should be getting your CYA adjusted to proper levels (300 is way too high) before adding any chlorine. After that is done, you should be adding chlorine. If there's nothing in the pool gobbling up FC, then yes, about 3/4s of a gallon of 10% bleach will raise it 3 ppm. If, however, your pool is unable to hold FC even though it has a proper amount of CYA, then you have ammonia/urea in your pool, at which point your "3ppm" will be gone overnight -- completely gobbled up by the ammonia/urea. In cases such as those, you have to add lots of chlorine in order to overpower the ammonia/urea... –  Michael Aug 7 '11 at 1:01
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A high CYA (cyanuric acid) will cause the chlorine levels to appear lower than they actually are. Unfortunately the only way to lower high levels of CYA is to add fresh water. You ideally want the CYA level to be in the 20-40ppm range. CYA is the stabilizer which slows down the process of chlorine evaporation due to sunlight. Trichlor tablets have stabilizer in them so you don't need to add any yourself. Likewise, shocking your pool with trichlor will also add stabilizer. CYA levels are usually lowered during the season via splash out.

There's nothing wrong with having high levels of CYA, you'll just have a hard time monitoring your chlorine levels. If you're close to the end of the season, I would say do nothing to correct the problem and then drain your pool when you close it, add some fresh water for the cover to sit on and let the rain fill it up in the spring. If you want to fix the problem now, drain your pool and refill it with fresh water. The amount to drain is dependent on the current level of CYA and the target level.

Another observation is that both your Total Alk and hardness levels look too high. TA should be around 100. Your hardness may be right on the verge of the high range. Personally, I'd bring it down closer to 300-350. Draining your pool and filling it with fresh water will accomplish this as well.

When it comes to adjusting chemicals, the Taylor recommended order is:

  1. Total Alk
  2. Calcium hardness
  3. pH
  4. Chlorine
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