Correct feeding is obviously important for the fish's nutritional requirements and the associated health of the fish.
The most common form of fish food is proprietary dried foods such as flake. These foods are designed as a 'complete' diet and should include all the vitamins, minerals, and other foods required for healthy body function. Despite being called a 'complete' diet, these foods should be supplemented with a variety of different foods such as frozen foods, which may help to increase the fish's health and encourage the development of better colours and breeding behaviour.
Feeding in the aquarium has a much larger and more important effect on the entire conditions of the aquarium though, which is often not apparent to new fish keepers.
The aquarium is an enclosed environment; anything placed in the water has a direct effect upon the waters conditions. Apart from the death of fish or plants, fish food is the sole source of waste products being introduced into the aquarium - whether it goes through the fish or not. The amount of waste that can be processed by the aquarium is dependant on the bacteria in the filter, which takes time to mature.
If the amount of food entering the tank exceeds the waste processing capacity of the filter, there will be an excess, which will break down and produce toxic ammonia and nitrite compounds. A physical excess of food (e.g. uneaten food) will not only add to this problem, but will also provide a habitat for disease causing pathogens (harmful bacteria and fungi) to spread.