The optimisation of feed composition is fundamental to maximising fish growth efficiency and performance in sustainable aquaculture. Traditionally this has been achieved through altering the quantity, ratio or type of macronutrients in fish feeds (i.e., proteins, lipids, carbohydrates). Here, we present an alternative approach that centres on reducing the energetic costs associated with digestion to enhance fish growth. Acid secretion to the stomach and recovery from the associated blood alkaline tide (rise in blood pH and HCO₃⁻ following the consumption of a meal) contribute to the energetic costs of digestion in fish. In this study, we tested the hypothesis that increasing the acidity of pelleted fish diets would reduce gastric acid secretion, the post prandial alkaline tide, the energetic cost of digestion and enhance the efficiency of converting food into growth. We examined the effect of three isocaloric diets of varying acidity on the growth, physiology and energetic costs associated with digestion in juvenile barramundi (Lates calcarifer). The consumption of an acidified diet in barramundi reduced the requirement for endogenous acid secretion to the stomach, prevented a post-prandial blood alkaline tide and reduced the total energetic cost of digestion by ~45%. This led to a ~ 14% improvement in the conversion of food into fish growth at 14 days. However, the observed improvements in feed efficiency declined over time. At day 28 there was no improvement in the conversion of food into fish growth in fish fed an acidified diet. This study shows that components that make up a feed that have no nutritional value can also be manipulated to improve fish growth efficiency, but further research is required to determine the long-term implications of feeding on an acidified diet in fish. This work provides a new avenue of investigation for fish feed formulation, sustainable aquaculture, gastrointestinal physiology and the energetic costs of digestion in fish.