Check any and all pool stores or buy it on-line. It can be called all kinds of things: Black Algaecide, Mustard Algaecide.

What I do is check the ingredients. If it says "Poly....<something unpronounceable>....60%" and the rest is "inert ingredients", that's the stuff.

Usually, at the end of the season, polyquat price drop. One store near me sells it for $30 to $35 during the season (absurd!) but around now they sell it for $15--and I buy a bunch because it keeps when it's in the basement.

I'm starting the closing process this weekend. But I won't finally close until the water gets close to or below 60 degrees. Until then, I'll keep the filter running and keep my FC up.

So, for now, I'm cleaning it, removing the solar cover, and the turning the solar panels off, so it cools.