Plenty of fresh water fish (carps in this case) are grown in ponds to which they add cow or pig manure. I guess it is to increase the amount of food for the fis

Plenty of fresh water fish (carps in this case) are grown in ponds to which they add cow or pig manure. I guess it is to increase the amount of food for the fish as microbes can feed on it and invertebrae can feed on the microbes. They have been doing it like this since 1500s.