Scientifically, this study aimed to validate the potential of ragworm to alleviate anaemia in common sole and to identify the dietary requirements of common sole to alleviate this anaemia. At the same time it was aimed to explain part of the difference in growth between sole fed a commercial pellet and sole fed ragworm. Practically, this study aimed to develop a diet that achieves similar growth rates in sole as when fed ragworm. Sole fed commercial pellets developed nutritional anaemia. Feeding ragworm or mussel alleviates this nutritional anaemia. It is suggested that the ability of mussel or ragworm (meal) to alleviate anaemia and improve growth in sole can be explained by heme iron and high B12 levels. Yet, iron absorption in sole is high and independent of iron source. Still, heme increases the absorption of copper. The high absorption of iron and copper in sole fed heme does not affect the haematocrit (Hct) and haemoglobin (Hb) levels, which indicates the anaemia in sole is not an iron or a copper deficiency anaemia. The Hct and Hb levels in sole are affected by dietary B12. Yet, the applied levels are unable to alleviate the anaemia in sole induced by feeding commercial pellets. More options to alleviate the nutritional anaemia in common sole are discussed. Nutrients as vitamin C, B1, B2, B5 and a possible role of dietary EPO are discussed. It is suggested that the slow growth of pellet-fed sole might be due to the low Hct and Hb levels, which hampers the uptake of oxygen, and thus also the overall metabolic capacity, including the scope for growth. Discussed is a 7°C difference in the “optimal” temperature between sole fed ragworm and the 2nd generation pellet and that the “worm effect” is dependent on temperature. However, the growth rate of sole fed the 3rd generation pellet at 18.4°C was comparable to the growth rate of sole fed ragworm, which could not have been the consequence of increasing Hct and Hb levels as these were comparable to levels found in sole fed commercial pellets. Yet, B12 levels in blood plasma of sole are up to 200 times those of other (fish) species. Hence, we inferred on the possibility of a specific metabolic function of B12 in respiration in sole. In addition, the economic and practical impact of the improved growth rate in sole culture is discussed. Finally, several suggestions for future research are given.