There is a saying "you are what you eat", and although it is generally directed at humans it probably applies equally to all sorts of creatures, even fish. The complication is, just like humans, fish are found all over the world, and have an equally varied diet. Not an issue if they are swimming wild in their native habitat and eating indigenous foods, but something of a problem when kept in a tank in the UK. Certainly processed fish foods have come a long way in the last 50 years, but are they sufficient to keep our specimens in good health throughout their lives?
The answer to this dilemma is probably yes, but only if used correctly and supplemented with suitable little extras. It is unfortunate that many fish keepers still only use the traditional flake foods as their fishes stable diet, with a little effort they could do so much better, and the first step is to understand your fish's natural diet.
Most fish are omnivores, they will eat practically anything, animal, vegetable or mineral.! Some do have preferences, but in general anything goes, including other smaller fish. That said, many do have some specific adaptations which the Aquarian should cater for if they are to keep their charges happy and there are numerous specialist products to assist with this.
Algae tablets are eaten with relish by most of the Catfish family and nibbled at by many others, and are a great source of vegetable material for the numerous herbivorous species. Frozen food has revolutionised the marine market, but is equally beneficial to freshwater speciesl. As well as Bloodworm (mosquito larvae) and Daphnia, which are both native to freshwater, saltwater invertebrates such Brine, Mysis and Gamma Shrimp, and even Krill, are all excellent foods for general use and very close to even a freshwater fish's natural diet.
Larger species will usually accept whitebait as an alternative to live food, although this may involve a long period of "Training". Members of the Tetraodon family (puffer fish) who normally eat shellfish will usually take whole cockles (in the shell). This has the added advantage of helping to keep their teeth in good condition.