This document specifies the laboratory test method for screening anti-fouling paints in a flow-through system using algae as the test organism. It is intended to be used in conjunction with ISO 21716-1, which specifies the general requirements of the test methods described in the other parts of the ISO 21716 series. The purpose of the test specified in this document is to determine the difference in colour of algae on painted test panels compared with algae on inert non-toxic control panels under the test conditions.