Effects of homochromatic lights on the physiological responses and morphochromatic traits of scarlet badis, Dario dario (Hamilton, 1822).

Aquarium fish keeping ranks as the world's second most popular hobby, with a global trade value of 18-20 billion USD annually. Despite its popularity, aquarium lighting is often used for aesthetics without considering its biological effects on fish. This study examined how homochromatic lights affect growth, feed utilization, survival, stress markers, hormone levels, and skin pigmentation in the ornamental fish Dario dario (scarlet badis). Ten male fish (92.79 ± 4.33 mg) were stocked per tank and exposed to six light treatments: red light (RL), blue light (BL), green light (GL), white light (WL), ambient light (AL), and dark (D) for 30 days under a 12:12-h (light:dark) photoperiod. Among treatments, GL significantly enhanced growth, feed efficiency, and survival. Stress biomarkers, namely superoxide dismutase activity, catalase activity, glucose, and cholesterol contents were significantly lower (P < 0.01) under GL, WL, and AL. Fish under GL also had elevated 11-keto testosterone and reduced cortisol levels (P < 0.01), indicating hormonal balance. Visibly, GL-exposed fish displayed more vibrant red bands, confirmed through skin chromaticity analysis and microscopic observation of pigment dispersion in chromatophores. In summary, green light created a favourable environment for D. dario, improving growth, stress reduction, and ornamental value.