Hi Ben. Thanks so much for the detailed response.

1. I did the CYA dilution test, as you suggested, and it did come in a little lower at 80. I also re-ran the regular non-diluted test and it was still over 100, but a little less than before. As I said, the pool store said I'm at 91; and the strips, while hard to read, seem to be dead-on at 100. In any case, my CYA seems to be somewhere in the range of 80-100.

2. Regarding borates, I have the LaMotte strips, which seem to be right between 30 and 50. Also, the pool store said I was at 29, and that was right before I added 2 boxes of borax, which should have taken me above 40. Are you sure I need 7 boxes of borax to go from 40 to 60ppm in 18kgal? According to the bleachcalc.exe, 7 boxes of 76oz borax (532 oz) would increase borate ppm by 90. I do recall reading here that the bleachcalc app might be inaccurate for borax. Just want to make sure before I dump that much borax in the pool.

3. I'll take the chlorine to 20ppm tomorrow after a trip to Wal-Mart to restock. Are you sure 20-30ppm is going to be high enough to kill this beast? From what I've read here, others have gone much higher (35-40 for 7+ days), and they had much lower CYA than me.

4. I'll get the OTO kit tomorrow also. I wasn't looking forward to counting 100+ drops in the DPD test.

I'll brush the pool regularly and update back here with any improvement. One question: because I have a cartridge filter, I can't vacuum this algae to waste. My pool store rents a portable pump for this purpose. Is that something I should consider also? Losing some water might also help drop the CYA a bit. Or should a cartridge filter trap this stuff? Do I need to clean the filters during this process?

Thanks again for the detailed response.