It is not clear whether animals require exogenous carnitine, and over the past 20 years, the scientific discussion regarding this subject continued with arguments pro and contra. Some studies observed that feeding fish with dietary carnitine supplements may improve growth and protect against several disease outbreak. However, such growth improvement was not always observed. The results can be interpreted in two ways: Firstly, the endogenous carnitine biosynthesis may be adequate to maintain sufficient tissue levels during growth. Secondly, the responses to dietary carnitine supplements are influenced by environmental and physiological interactions. The aim of this mini review was to provide an overview of the positive effects of dietary carnitine supplementation on fish metabolism, and possible factors that might alter dietary carnitine requirements, such as feed composition, species, age, tissue dependence on fatty acid oxidation and metabolic conditions.