Can't keep chlorine level

[Stumped]

I've been unable to get a chlorine level all summer. I'm pretty fed up and now just want to get the water right so I can throw on the cover. I have a 17,000 gallon above ground (27' round) pool. Here were my readings yesterday from a pool store analysis:

FC- 0
PH- 7.2
TA- 90
CYA- 30
Phosphates- 200

I added about a pound of PH increase last night. No matter how much chlorine or bleach I put into the pool it keeps getting eaten up. I had a bad algae problem when I returned from vacation a week and a half ago but treated it, vacuumed, backwashed several times and now the water is blue and fairly clear, somewhat cloudy. Last night I put 2 gallons of 5.4% bleach in the pool and still read 0 so I put 2 more gallons in. Initially last night I saw a reading of 3.0 (the first time all year) but by this morning it was back down to 0.5%. Should I just keep throwing more bleach in to get my levels up or is there something in my water which will just continue to eat it up?

[chem geek]

If you are using a DPD chlorine test where you see the intensity of red/pink compared to a standard, then this test can get bleached out above 10 ppm FC making you think you have no chlorine when in fact you have a lot. That's one possibility. If you use an OTO chlorine test where you see the intensity of yellow (to orange with high FC) then this test will not get bleached out. The best test is the FAS-DPD chlorine test which can measure accurately up to 50 ppm and is in the Taylor K-2006 kit you can get at a good online price here or the TF100 kit from tftestkits(dot)com here.

If you've made sure that the chlorine reading is correct, then you can do a bucket test to see if you can get the chlorine to hold. You can also get an inexpensive ammonia test kit from a pet/fish/aquarium store since it's possible that during the algae bloom some soil bacteria converted some or all of your CYA to ammonia. It takes nearly 10x the amount of ammonia to clear it with chlorine as FC so it may be worth a partial drain/refill instead if that is the case.

Richard

[Stumped]

Thanks. The FC test is accurate as I've had it tested many times at pool stores. If I have to drain and refill how much is recommended. Like I said at this point I'm just trying to get the water ready to be covered for the winter. Ammonia is a possibility, I've also heard discussion that suggest it could be nitrogen, possibly from lawn fertilization? Is there a test for that and would that also require a partial drain/refill?