Hi and welcome.
First, Phosphate levels are rarely a problem. We have users with levels over 3000 with clear, sanitary pools. Phosphates FEED algae, but if you don't have any algae anyway, they can't feed and grow off it. Phosphates are simply the latest "gimmick" in pool store sales.
Your problem is multiple.
First, your pH is way too low. You need to add Borax immediately, right now, especially if you have a vinyl pool. What size is it? What kind of filter and pump?
Borax will safely raise your pH. If your pool is 10,000 gallons or more, add a full box. Wait an hour, test pH, and repeat, until pH is at least 7.2. If it's under 10,000 gallons, use a half box. If it's under 5000 gallons, use 1/4 box....this is just ball-parking. Yes, 20 Mule Team Borax. From the supermarket's laundry section.
DO NOT USE POOL STORE PH RAISERS! THEY WILL RAISE YOUR TA TOO MUCH! Besides, all the pool store products like pH Up! and Balance Pak 200 are nothing more than Arm&Hammer Washing Soda--Sodium Carbonate.
If you have a vinyl pool, a pH below 6.9 can damage your liner. That is more important than algae so it needs to be fixed.
Next, when your pH is fixed, you can worry about the next problem: Algae.
For your level of CYA, you need to raise your FC level to the shock level, which is 15ppm, and keep it there. DO NOT USE TRI-CHLOR OR DI-CHLOR PUCKS OR POWDER! THEY ARE ACIDIC!
We recommend ordinary 6% bleach for chlorination. 1 gallon of 6% adds 6ppm of FC to 10,000 gallons of water. Figure accordingly. If your Calcium level is fairly low (200 ppm or less) you can use Cal-Hypo powder. It tends to raise pH a little, can cause white cloudiness (not a major problem) and adds calcium. But bleach or Liquid Chlorine (same thing. Most LC solid is stronger, about double) is best.
You need to test and add more bleach at 3x/ day: Morning, mid-day to dinner, and after sunset. You want to maintain FC at 15 so that it doesn't drop much over night.
Plus you'll need to run your filter 24/7, and brush your pool every day, and vacuum to waste at least once a day if you see debris on the pool floor.
Check out our sister site at poolsolutions.com. It's got lots of great information that WORKS!
All the best,
Carl
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