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Astaxanthin is a naturally occurring anti-oxidant, best known for causing the red colour in shellfish. Although the chemical astaxanthin can be synthesised chemically, the most common source of naturally occuring astaxanthin is krill, microalgae and shrimp. Because of its health benefits and classification as an anti-oxidant, fish keepers who make their own foods recommend adding it to recipes for increased fish health. It is available as a supplement or in aforementioned krill, shrimp and algae for inclusion in a recipe.