Physiological and growth responses of common carp ( <i>Cyprinus carpio</i> ) to dietary onion skin ( <i>Allium cepa</i> ) powder supplementation

Onion (Allium cepa L.) skin, a polyphenol-rich agro-industrial by-product, has potential as a natural additive in aquaculture. This study investigated the effects of onion skin (OS) powder administered through diet or rearing water on growth performance and physiological responses of common carp (Cyprinus carpio). Fish (118 ± 5 g) were distributed to five groups: control, dietary OS at 5 and 10 g kg−1 and waterborne OS at 5 and 10 g tank−1 for 56 days. OS supplementation significantly improved final weight, weight gain, specific growth rate, feed efficiency and selected organ indices (p < 0.05), with the best performance observed in the high-dose dietary group. Hematological and serum biochemical parameters were also improved, with significant decreases in glucose, cholesterol, triglycerides and low-density lipoprotein levels. These findings suggest that OS powder may serve as a sustainable, natural additive to improve growth performance and physiological condition in common carp.