Killing algae requires the appropriate chlorine levels which are based on your CYA level. If you will take a look at this chart http://pool9.net/cl-cya/ , you'll see that with a CYA of 45ppm, your shock level is 15ppm. You need to raise your chlorine to that level and hold it there until you are losing no more than 1ppm of chlorine from sundown one evening to within one hour of sunrise the next morning. At that point, we advise holding the chlorine high for one additional day and then let it drift down but always keep it between the minimum and maximum in the chart or else you'll end up getting algae again.
The problem is that you don't have a kit that can measure chlorine readings that high and we have learned through the years, that most pool store testing is inaccurate. It looks all official with the readings to two decimal places but it usually really isn't. That is why we advise getting a good kit for yourself so you don't have to rely on pool stores. The test kit that we think is by far the best is the Taylor K2006 or 2006C (better buy). Not available locally but you can get it through this link that takes you to Amazon: http://pool9.net/tk/
What kind of kit do you have? One that uses a yellow scale to read chlorine? If that is the case, you can use the chart at this link to read off the scale OTO readings:> http://pool9.net/oto-chart/ This does not negate the need to buy a good kit but just gives you something to use in the meantime. (Not super accurate though.)
A CYA reading of 45ppm is just about perfect. So, you don't want to use any forms of chlorine that have CYA in them such as trichlor tabs or dichlor powder. Best to use is just plain, unscented bleach. Many of us use Walmart's generic 8.25% bleach.
Test in the evening and add enough bleach to take your chlorine level back up to 15ppm. In a 3800 gallon pool, each cup of bleach will add approximately 1.5ppm and each quart will add about 5ppm. You can use those as a reference to help you figure out how much bleach you need to add to get back to shock level.
Your pH test won't be accurate when the chlorine is over 5ppm (or over 10ppm with a Taylor kit). So, if you need to test pH when the chlorine is high, you'll need to dilute the sample first by mixing one part pool water with an equal part of distilled water and then run the pH test with that mix as normal.
Keep running the pump 24/7 and clean the filter as needed and also brush the pool daily.
Hope this helps.
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