Early environmental conditions play a critical role in shaping fish physiology and behaviour, with tank colour emerging as a relevant yet often overlooked factor in aquaculture and experimental research. This study investigated how rearing tank colour affects larval performance in the cichlid fish Cichlasoma dimerus. Larvae were reared in white, light-blue, or grey tanks, and survival, growth, pigmentation, sex ratio, skeletal development, and the endocrine responses of somatolactin (Sl) and growth hormone (Gh) were evaluated. Survival was significantly lower in white tanks. Conversely, larvae reared in white tanks reached a higher final body weight, while total length showed a similar but non-significant trend. Gh-immunoreactive cells exhibited significantly larger nuclear areas in larvae reared in white tanks and a tendency toward smaller cytoplasmic areas compared with those from light-blue tanks. Melanophore number did not differ among treatments, whereas larvae from grey tanks showed a higher number of Sl-immunoreactive cells. Sex ratios tended to be female-biased in white tanks. Skeletal development did not differ among tank colours and followed the expected chondrogenesis and ossification sequence. Overall, tank colour influenced multiple aspects of larval development, particularly survival, growth, and endocrine responses, underscoring its relevance in fish rearing and experimental design.