Long story related to CYA way over 100 all of last year due to years of puck usage by previous owner, even after draining and replacing ~1/3 of the water, 2+ PPM of iron and TA of ~240 all of last year, resulted in draining and replacing ~98%+ of the water last week after using AA to remove rust staining. Since my well water has 5 PPM of iron, TA = 450 hard and TA = 550 when softened, I used lake water to replace the pool water. I had tested it at TA 180 out of the lake, but it's currently closer to 200 in the pool.
All of last year the PH was actually quite stable given the high TA and I never had to adjust it as it was steady at around 7.5 - 7.8.
Current test results
TA = 200
CH = 150
CL = 3.5 (still coming down from slamming to 12 after filling with lake water)
CC = < 0.5
PH = 7.2 The Iron unfortunately is just under 1 PPM even from the lake, so figured it's better to leave the PH lower, and it's been steady for a week, even with the high TA.
CYA = < 10
I'm not concerned about the CYA at this point as the pool is lightly used and has an automatic cover so there's little CL loss to sunlight. I can bring it up over time if I decide to do so with pucks while on vacation, etc. CH is fine as I have a fiberglass pool that doesn't have any calcium in the gelcoat, so I don't really want to raise it. Less chance of lime deposits with lower hardness.
The pool is looking very good now after a week of 24 hour a day filtering. It was very green to start with, I assume from algae in the lake water, but it's been killed and removed, and other than some debris that's settled on the bottom that I need to vac again, it's nice and clean.
Due to the automatic cover I'm not keen on using acid to lower TA and aerating to raise it back up again, primarily because I don't live there and don't like leaving the cover open with no one around. There's no fence around the pool, so it's not a good idea from a liability standpoint.
In addition, I tried this last year with almost no success. I aerated for 3+ days for 24 hours continuously with the cover partially open, and the PH @ ~7.0, and the PH would not come up after dropping it with acid and days of aeration. I finally gave up and put Borax in to raise it up again, and let it go. Interestingly enough, the rest of the summer I never had to add either acid or Borax to adjust PH again and it remained steady where I adjusted it. I made an aerator with PVC pipe and holes drilled in a cap to bring it up above the water and spray back down into the water. It created lots of foam, but didn't do squat for the PH.
I'm a bit anal, and I would prefer the TA to be ~100, but I'm not sure I can get there, plus with 550 TA from the softener, anytime I add additional water it will drive it up again. I most definitely won't be using hard water to refill due to the high iron content. I would also like to use Boric Acid to Borate the pool, but I understand that it's important to adjust the TA first, assuming I can get it adjusted. However, if aerating won't bring up the PH, it becomes a matter of using acid to lower the PH & TA and using Borax to raise the PH again. However, I'm not sure that this will even bring the TA down while maintaining an appropriate PH level.
Given all the above, how important is it to try to lower the TA?