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Formulated feeds are nutritionally complete pellets that are formulated for specific fish and life stage (Figure 15). Unlike other animal crops in agriculture, the nutritional needs of fish vary greatly among species for protein, fat, and carbohydrate inclusions. A carnivorous fish who eats at the top of its food chain, like a largemouth bass, requires a diet with high protein and low carbohydrates. On the other hand, omnivorous or herbivorous fish, like catfish or tilapia, require less protein and can tolerate higher levels of carbohydrate in their diets. This is important in aquaponics because the nutrient composition of the feed pellet drives the nutrient load available to the plants. As the fish feed is consumed and excreted by the fish, nutrients are released into the water as dissolved or solid particulates, which get circulated and used for plant growth. For example, feeds with a higher protein content will deliver a higher amount of total ammonia nitrogen (TAN) to the system, as nitrogen is primarily derived from the protein in the feed. The amount of TAN produced from a particular feed per day can be calculated using the following formula from Timmons and Ebeling (2013):

PTAN = Feed input (g) x Protein content (%) x *0.092 ÷ time

*0.92 represents = 0.16 x 0.80 x 0.80 x 0.90

Example calculation for 2,000 g feed per day at 32% protein:

PTAN = 2,000 g x 0.32 x 0.092 ÷ 1 day

This rate is equivalent to approximately 3% of the feed rate per day.