ATtack is an amazing biocide with incredible features. It will kill algae and other micro organisms such as spores, fungi, and prevent regrowth for at least a year.
It works in wet environments and continues to work after drying. It has very low toxicity to both humans and animals, has a low environmental impact, and is highly biodegradable.
It is easy to use, available in a 5l container, which diluted 10:1 with water will treat 400sq m broadcast with a garden or back pack sprayer.