The research around fish and the production of fish for consumption is growing rapidly.  Since the common carp is a major source of food for human in many parts of the world there are many studies becoming available on how those in aquaculture can become more successful and productive. In this study Quercetin, a plant pigment (flavonoid) and supplement that is found in many plants and foods, such as red wine, onions, green tea, apples, and berries is tested for its use supporting carp as they grow.
 
"In this study, the dietary incorporation of quercetin significantly improved growth and feed efficiency in common carp. Accordingly, the best WG, SGR, FI, and FCR were obtained in the fish fed with 400 and 600 mg/kg quercetin-supplemented diets. Previous findings indicated that dietary quercetin had a positive effect on the WG, SGR, and FCR in Nile tilapia (Oreochromis niloticus) [34], grass carp (Ctenopharyngodon idella) (), and common carp [13]. The promoting effect of quercetin on fish growth performance may be related to their better efficacy in stimulating the secretion of digestive enzymes like protease, amylase, and lipase [13, 34], decreasing the abundance of harmful bacteria of the intestine and promoting the populations of beneficial bacteria [35], ameliorating the intestinal barrier function, and improving the surface area of intestinal villi and goblet cells [36]."
 
https://www.hindawi.com/journals/anu/2023/1168262/