What is the fine-grained topic of the following text: Moulds says, “We raise fish to produce nutrients in the water of which bacteria converts the waste from the fish into usable nutrients and provide the food for the plants.” The fresh water fish are tilapia, the 6th most common seafood product in the U.S. They’re being raised as part of the 9,000 gallon closed system. Only return the topic.
Raising tilapia in a closed system aquaponics