3d,

Please read the "Best Guess" page, linked in my sig and Watermom's. Your statement that chlorine won't kill algae, regardless of level is simply wrong.

Once you've read that page, if you want to know more, look at some of Chem_Geek's posts in the "China Shop".

Algae guard is just a plain quaternary ammonia algaecide. They are as a class, irritating, foaming, and unstable in the presence of chlorine. They do kill algae, but there are better ways to do that.

Ben