Wow!
You've done a GREAT job! What you've done is exactly right. Draining your pool and bumping FC up to 20 with a CYA in the 50-60 range (you don't really know it more than that) will clear it.
The only problem I have with lowering TA at this time is that with an FC of 20 your pH is probably lower than it looks. When you measure it, try add two drops of 0007 first--to neutralize the chlorine in the sample.
Meantime, you CAN drop your pH but given what I've said, I'd go no lower than 7.2--below 6.9 can start to damage the vinyl so be safe. This will bring the TA down some. Then aerate the water to raise the pH back up, add more acid to bring it down to 7.2, etc. This ratchets the T/A down. When TA gets down below 200, you MAY (no guarantees) see the water clear.
As a bonus, at the lower pH the chlorine is more effective.
But don't rush things--take it slow.
Also, are you backwashing everyday? You may also want to add a little DE powder via your skimmer to your sand filter to improve filtration.
Add about 1/3 cup and wait 20-30 minutes. The goal is to increase your pressure by about 1 psi. If it hasn't gone up, add another 1/3 and wait again. When it hits 1 psi, you'll know how much to add next time. Every time you backwash you can add this amount of DE. It should help.
Buy the smallest box--KMart or Walmart or even Mr. Cold Shoulder should have 10lb boxes--you won't need more as that will last you a couple of seasons.
But keep doing what you are doing. Obviously it's working!
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