What Watermom said.

If you DO add PolyQuat, remember that your FC may drop drastically. My FC was 4 last week. I added a quart and 1/2 of PQ and 24 hours later FC was 0. Not a worry, I added Chlorine. Yesterday, 0 again. So I added 3 gallons of 12.5% LC--today FC was 9 (But I was aiming for 15). So since I noticed my CYA was at 30, I added Di-Chlor instead...tomorrow I'll check again.

But the drop was NOT due to algae, it was due to excessive PQ--I expected it, and so should you.

BTW, I go through this every closing--and open to clear, clean water in the spring, every year.