I have to make one addendum to Sean's post.

I have a quart container in front of me of Blue Shield "Mustard Algae Eliminator" made for NAMCO that contains pure Polyquat 60%

It's active ingredient is....: Poly (OXYETHYLENE(DIMETHYLIMINO)ETHYLENE(DIMETHYLIMINO) ETHLYENE DICHLORIDE)....60%

Inert ingredients: 40%.

That's why we say if it says "Poly...<something,something> ...60%" it's PolyQuat.

The moral is: No matter WHAT an algaecide is called, read the ingredients. If you don't see "Poly...<something>...60%" as the only active ingredient, DO NOT BUY IT!