I want to thank troublefreepool community for helping me fix my pool algae problems. For those interested, here is my story:
I bought my current house with a pool 2.5 years ago. I had never had a pool before, so I continued using the trichlor pucks that were there when I moved in. I checked pH and chlorine with an OTO test occasionally, and had no problems until last fall when my filter broke while I was on vacation.
We called the local pool supply store, which sent a tech to fix the filter. The repair didn't get done for a week, we had a bunch of rain storms, and I ended up with algae growing on the walls and bottom. Went to the pool store, and was sold Green to Clean and dichlor shock. This did get rid of the algae. However, even after the filter got fixed the algae came back. I stupidly assumed we just didn't get it all and went back to said pool supply store and got more green to clean and shock. This again fixed algae for week or so, and then it came right back.
At a high level, I understood why green to clean worked. It contains ammonium sulfate, which promptly reacts with the chlorine from the dichlor shock to give chloramine, which is quite toxic to many living organisms. I understood trichlor and dichlor release HOCl and CYA. I naively assumed that CYA must break down in the pool somehow (pushing a few electrons allows one to draw a hydrolysis of cyanuric acid with 3 moles of water to give 3 moles each of NH3 and CO2).
I was wrong, of course, but thankfully googling for solutions to pool aglae led me to troublefreepool.com. The excellent chemistry discussions here made it precisely clear why I was having problems: my CYA was so high from years of trichlor and the ill-advised shocking with dichlor that I couldn't get a high enough concentration of HOCl to kill anything. Adding NH3 gave high chloramine levels which worked for a bit, but wasn't a permanent fix. Shame on the pool store people - they should know better.
At any rate, after measuring CYA with the TFT test kit (around 200) and replacing most of my water I now have CYA at 60 ppm. I shocked with Costco bleach and had FC drop from 24 to 1 in 2 hours, and CC rise up to 12 or so. Clearly lots of stuff to kill. I kept the FC up for a couple days and got things nicely under control (FC stabilized at 12, and CC undetectable after 2-3 days).
Since then I have easily maintained FC at 5-10 with bleach and have beautiful blue water with no more algae. Thanks to the troublefreepool community for the informative posts, pool chemistry lessons, and the amazing pool calculator.