Here’s a quick rundown of some of the most widespread offerings.

Quats plus copper

Some manufacturers offer useful options for treating black algae, such as combinations of quaternary ammonium compounds — “quats” — with metals such as copper, which disrupt enzymatic activity within the algae cells.

“The quat acts as a wetting agent or a penetrant, and it’ll actually help draw that algaecidal activity into the porosity of the plaster, so the copper algaecide actually gets to those deep-set cells better,” says John Puetz, director of technology at Advantis Technologies in Alpharetta, Ga.

Blended polyquats

Just as polymeric quats, or “polyquats,” offer many of the advantages of quats without a tendency to foam, blended polyquats include additional ions — electrically charged atoms — that attract and hold the negatively charged cell membranes of dead algae cells. These compounds not only kill algae, they also help streamline the whole cleanup process.

Phosphate treatments

Many manufacturers offer phosphate removers, which help clear out the phosphates on which algae feeds. However, some recent products combine phosphate removers with other types of algaecides, such as quats or metals — thus minimizing available nutrients for algae while simultaneously killing any cells that remain.