I'm guessing your volume to be around 13,600. With 54" walls? your water level is more like 4 ft or maybe 4 '2." If you add that amount of bleach, that will add roughly 6.5ppm of chlorine which would take you up to around 7ppm.
But....... when you run the CYA test and get a result of 100, it means that your CYA is 100 or higher. Either way -- 100 or 200 or whatever, your FC reading of 0.5 is WAY too low. Needed chlorine levels are based on CYA readings. The higher the CYA, the higher the chlorine has to be kept or else you risk an algae bloom. More about the CYA/Chlorine connection can be found here:> http://pool9.net/cl-cya/
Your chlorine level in your pool with this very high CYA reading should be kept between 8-15 ALL the time. Since it dropped so low, you may have an impending bloom that you don't yet see so you need to shock your pool. Your shock level will be 25ppm. Each 121-oz jug of 8.25% bleach will add just shy of 6ppm. This evening, go ahead and add about 4 jugs of bleach.
Here's another problem. Your 6-way kit can only read chlorine levels up to 5ppm. We typically tell people to buy a Taylor K2006 kit which can test higher chlorine like you are going to have to run. But, since you have the HTH 6-Way kit, if you just pick up a Taylor K1515 or 1515C kit, it will give you the missing FAS-DPD chlorine test you need. You can get it through this link:> http://pool9.net/tk/
In the meantime, you'll have to rely on your OTO chlorine tester. This chart will help you interpret the off the scale chlorine levels. It is not super accurate but will have to suffice in the meantime:> http://pool9.net/oto-chart/
You want to keep your chlorine at shock level until you can go from sundown one evening to within an hour of sunrise the next morning without losing more than 1ppm of chlorine. Once you pass the overnight test, let your chlorine drift down but keep it between 8-15 all the time.
Go ahead and add a quarter of a box of Borax. Once your chlorine is higher than 5ppm, you won't be able to test pH without diluting the sample first. Mix one part pool water to an equal part distilled water. Mix and then run the pH test with that sample. (On the Taylor kits, the pH test is good up to chlorine levels of 10ppm.) A pH of less than 6ppm is really bad for pools and equipment. Any reading under 7.0 is acidic. Those trichlor tabs are the culprit. They are extremely acidic and will drive the pH down.
TA and CH are fine.
What are the ingredients of EZ-pool? How much trichlor do you have left and are the tabs wrapped or unwrapped?
I know all this is way more than you asked for but ....... there is a lot of information that you need to correct what is going on in your pool. Your water may look clear but super low pH and high CYA and low chlorine are all things that can cause potential problems. Hoping to help you turn things around.
Welcome to the Pool Forum, by the way!
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