INTRODUCTION

The ornamental fish industry is one of the largest sectors of the global aquaculture trade, with an estimated annual market value of US$15–30 billion [1]. However, intensive commercial production poses significant challenges to maintaining fish welfare under increasingly demanding production systems. Ornamental fish are exposed to multiple stressors, including transportation, handling, high stocking density, increased feeding intensity, and intensive management practices compared with extensive production systems [2, 3]. These stressors induce physiological disturbances, impair growth, reduce skin pigmentation, and increase susceptibility to bacterial, viral, and parasitic infections, ultimately resulting in disease outbreaks and substantial economic losses in commercial aquaculture. Moreover, compromised health and chronic stress adversely affect overall fish welfare. Global restrictions on the use of antibiotics and synthetic chemicals for growth promotion and disease prevention have emerged due to concerns about antimicrobial resistance and their adverse effects on aquatic ecosystems and animal health [4]. Consequently, there is an urgent need to develop sustainable nutritional strategies that improve fish health, disease resistance, and production performance while reducing dependence on synthetic compounds.

Natural immunostimulants have emerged as promising alternatives for enhancing fish health and preventing infectious diseases in aquaculture. These immunostimulants include prebiotics, probiotics, and medicinal plants [5]. Herbal feed additives are widely available and can be incorporated into aquafeeds as whole plants, specific plant parts, or purified extracts. Their natural origin, environmental compatibility, ease of application, and favorable effects on fish welfare make them attractive candidates for sustainable aquaculture [6]. Furthermore, medicinal plants contain diverse bioactive compounds that promote growth, exhibit antimicrobial and antioxidant activities, stimulate immune responses, and improve stress tolerance [5, 6, 9]. Owing to these beneficial biological properties, herbal products have attracted considerable attention as functional feed additives that support sustainable and environmentally responsible aquaculture practices.

The gac fruit (_Momordica _cochinchinensis) is a native Vietnamese fruit belonging to the family Cucurbitaceae [7]. The fruit contains bright red, soft, and sticky arils approximately 1–3 mm thick [8]. Gac fruit contains approximately 5% crude protein, with abundant leucine, arginine, and glutamic acid, and 1%–2% crude fat, whereas the arils contain a substantially higher lipid content of approximately 22.3% [9, 10]. In addition, gac arils are exceptionally rich in bioactive compounds, including α-tocopherol, provitamin A, fatty acids (oleic, palmitic, and stearic acids), β-carotene, lycopene, polyphenols, and flavonoids [11–13]. These compounds contribute to the remarkable biological activities of gac arils, including antioxidant, antimicrobial, anticancer, and anti-inflammatory effects [14, 15]. Because of their exceptionally high phytonutrient content throughout the aril, seed, pulp, and peel, together with their recognized medicinal and pharmaceutical properties, gac fruits have been referred to as "super fruit" and "heaven's fruit" [16]. Among these tissues, the red aril oil represents the most nutrient-dense fraction because it contains abundant oil and exceptionally high concentrations of carotenoids, including β-carotene, lycopene, cryptoxanthin, and zeaxanthin [12, 15]. Recent efforts have focused on developing gac aril powder (GP) and gac aril oil (GO) as natural functional food ingredients and medicinal products. Furthermore, the naturally high lipid content of gac arils facilitates the absorption of carotenoids and other fat-soluble nutrients [17–19]. Consequently, GP and GO have considerable potential as sustainable natural alternatives to synthetic carotenoids in ornamental fish diets while simultaneously improving pigmentation and commercial value. Despite extensive investigations in terrestrial animals, studies evaluating GP and GO in aquaculture remain scarce. To date, only one study has demonstrated that dietary gac ethanol extract enhanced pigmentation in false clownfish (Amphiprion ocellaris) [20].

Goldfish (Carassius auratus), members of the family Cyprinidae and order Cypriniformes, are among the most widely cultured freshwater ornamental fish worldwide. Their commercial value is largely determined by body coloration, with bright red and orange pigmentation representing highly desirable market characteristics [21, 22]. Because goldfish cannot synthesize carotenoids de novo, dietary supplementation is essential to maintain and enhance skin pigmentation [21, 22]. This physiological limitation provides an opportunity to improve fish quality through nutritional intervention. Previous studies have demonstrated that dietary supplementation with natural carotenoids and bioactive compounds derived from medicinal plants enhances pigmentation and improves the overall health status of goldfish [23, 24]. Conversely, carotenoid-deficient diets result in progressive fading of skin pigmentation. Moreover, because of their manageable size and ease of maintenance in vitro, goldfish are widely used as experimental models in biological research [25]. Given their economic importance in the ornamental fish industry, improving growth performance, coloration, and health through sustainable nutritional approaches remains a priority. Natural herbal feed additives therefore represent environmentally friendly alternatives to synthetic pigments and chemical supplements for ornamental aquaculture.

Although _M. _cochinchinensis has been extensively investigated in humans and terrestrial livestock for its exceptionally high levels of carotenoids and bioactive compounds, its application in aquatic species remains largely unexplored. Existing studies have primarily focused on the nutritional composition and biological activities of gac-derived products or evaluated crude extracts in limited fish species, leaving substantial gaps in understanding their efficacy as functional feed additives in ornamental fish. Furthermore, no previous investigation has systematically compared the biological effects of GP and GO, despite their distinct physicochemical characteristics and potentially different carotenoid bioavailability. In addition, comprehensive evaluations integrating growth performance, pigmentation, carotenoid deposition, hematological and serum biochemical responses, liver histology, and immunity-related gene expression within a single experimental framework are currently lacking. Addressing these knowledge gaps is essential for determining whether different forms of gac-derived products can serve as effective and sustainable alternatives to synthetic carotenoids and chemotherapeutic feed additives in ornamental aquaculture.

Therefore, the present study aimed to comprehensively evaluate the effects of dietary supplementation with GP and GO on growth performance, feed utilization, body pigmentation, tissue carotenoid deposition, hematological and serum biochemical parameters, liver histology, and immunity-related gene expression in goldfish (C. auratus). In addition, this study directly compared powder and oil formulations of gac arils to determine their relative biological efficacy and potential differences in carotenoid bioavailability. We hypothesized that dietary GP and GO, particularly GO, given its greater lipid-mediated carotenoid availability, would enhance pigmentation, improve physiological and immune responses, and promote overall health without adversely affecting growth performance or liver integrity, thereby supporting their application as sustainable, multifunctional feed additives for ornamental aquaculture.