The Heart is a pumping device that pumps blood.
The heart of a fish is a two-chambered heart. It has one Auricle and one ventricle.
Blood from all the parts of the body is sent to auricle, auricle pumps that blood to ventricle and ventricle pumps that blood to gills for the purpose of oxygenation.
From the gills, oxygenated blood is supplied to all the parts of the body.
Hence, we can say that fish heart is a single circuit heart as blood passes through the heart only once in a circulation.