They also serve a variety of supporting functions in body chemistry. Fish extract some calcium from hard water, but fish kept in soft water need calcium in their food. If an aquarium is left unplanted, then natural forms of phosphorus from the plants are unavailable for fish to eat. Many minerals are poisonous if they are present in excess, so mineral supplements should not be added to the food indiscriminately. Some fish may fail to prosper when denied their regular foods required of that species.
Nutritionists are able to select ingredients on the basis of known nutritional content.
Not all nutritional factors have been identified yet. For example, some fish like scats, that feed heavily on algae or other plants, languish in captivity if vegetable is deleted from their diet. Leafy vegetables such as spinach or lettuce are suitable for some fish but not all. I must add a personal comment about my dad's fish. These fish do not spend alot of energy finding their food.