A couple of comments, do you have iron stains on some of your fixtures in the house (toilet, sinks, shower?)I'm on city water, not well water. I'll take a sample of my pool water and the tap water in for testing as soon as I can. I'm thinking with all of the rain we had in Jan/Feb, maybe runoff caused the high iron?
My biggest concern with draining 1/2-3/4 of the pool is the potential flooding of my back yard as well my neighbors. That's a lot of water. I'd need about 4000' of backwash hose to run it out to the ditch line near the road. How do others handle this?
I'll be ordering 5lbs of ascorbic acid and start that process after I get the water samples tested, but 99% sure it's iron, I was able to clean the steps and a good section of wall and some floor with a bottle of vitamin c.
Having neighbors and 4000' from the street are two interesting combinations for sure. If you have city sewer, is there a cleanout that you can dump into?
Lets see how the tests come back before we offer some more options. The key will be the tap water results. I wouldn't think that you would have enough iron rich water running off into the pool to cause that kind of staining.
Can you post a picture of your pool/water.