In The Swim Black Algaecide – Eradicates Black Algae in Swimming Pools - Copper Sulfate Pentahydrate - 1 Quart

Copper ions from copper sulfate are used as algicide for things like garden ponds. The problem is that it doesn't evaporate like chlorine so it builds up in the water and changes in pH can cause it to fall out of solution. They sell chelated copper algicide, which helps avoid these issues, but usually, you still need chlorine.