Giant grouper (Epinephelus lanceolatus) is an economically important yet under-researched species, still reliant on ‘trash fish’ or aquafeeds that do not meet its nutritional requirements. As a key step towards formulating a species-specific aquafeed for giant grouper, we have sought to define appropriate dietary levels of methionine, sulphur amino acids and related compounds. This nutrient complex is often limiting in feed formulation and underestimation of their interactive requirement values is a major limitation in generically formulated marine fish feed. Interactions between related metabolites and the formulation considerations surrounding these interactions will be discussed.