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Typical diet items for fish in aquariums include a variety of commercial pellets and flake foods, gel diets, harvested fish and aquatic invertebrates, plants and algae, and live foods. Nutritional analyses for common food items fed to fish are presented in Tables A4.5–A4.8.

Animal‐based diets, particularly fish‐based diets, often show higher digestibility and better fatty acid and protein profiles for fish, but there may be concerns about high total fat content, cost, and sustainability (Cho and Bureau 2001). Efforts have been made to increase the use of plant‐based fish diets. However, plant‐based diets may contain anti‐nutritional factors (e.g. phytate, non‐starch polysaccharides (NSPs), protease inhibitors). These can reduce digestibility and impair fish health, depending on adaptations to these diets, processing of the ingredients, and enzyme additions (e.g. phytase) (Kumar et al. 2012; Lemos and Tacon 2016).

Undigested feed can negatively impact water quality in the facility and after water is released. The major concerns with fish are solid wastes, nitrogen, and phosphorus, which can contribute to eutrophication (Cho and Bureau 2001). Nitrogen tends to be of greater concern in seawater, whereas phosphorus tends to be of greater concern in freshwater, as they are essential for algal growth in the respective environments. Most of these wastes come from the diet, so improving food types can reduce environmental impacts. Nitrogenous waste can be minimized by providing optimal levels of amino acids and reducing the digestible protein:energy ratio (McGoogan and Gatlin 1999). Phosphorus waste may be the result of variable P digestibility, and/or from phytate‐bound phosphorus in plant‐based ingredients. In the latter situation, the addition of phytase enzymes may reduce phytate‐P excretion by fish species which digest little to no phytate‐P autoenzymatically (Papatryphon and Soares 2001; Hua and Bureau 2010).