Dover sole (Solea solea) which are farmed in an aquaculture system such as sea ranching show a slower growth rate and the system has a lower productivity than expected from sole living in a more natural marine environment. The addition of a source of heme and optionally a source of vitamin B12 to the diet of the fish results in an increase in daily growth rate and therefore an increase in productivity of a fish farming or ranching system. The addition or heme and vitamin B12 to the diet has the effect of reducing an anaemic growth suppressing effect of normal commercial fish food diets on sole. Medicaments for fish for preventing or treating anaemia and to increase haematocrit and haemoglobin include heme and optionally also vitamin B12.
