What to feed fish
Fish are, like all Life on Earth, carbon-based lifeforms, and as such they eat carbon-based food.
In nature fish have narrower eating habits, but when kept in aquariums they accept a surprisingly wide range of food types. For example, a fish that is completely herbivore in nature, would be happy if given meat and live food in an aquarium.
In fish stores you can buy balanced flake foods for any kind of fish.
It is a common problem that many beginning aquarists overfeed their fish. It needs to be said that fish need very little food. The first reason for that is their size. It is obvious that fish are much smaller than us, and they eat very little. The second reason is that fish, unlike humans, don't waste an enormous amount of energy to keep their temperature steady.
They also don't know when to stop eating, and excess food simply passes through them and goes out through the other end. This contributes to water pollution, and fish waste eventually decomposes into toxic substances, which leads to smelly water and/or ammonia poisoning.
There are also edible things (eg algae) naturally occur in an aquarium. By not overfeeding your fish, you're encouraging them to make use of these food resources too.
Adult fish should be fed twice a day. It is undesirable to feed them only once a day, especially very late in the evening.
If you manage to be regular in feeding, your fish will more easily learn that you are the one who feeds them, which is the only way to get them to like you.
Most fish really like live food. Flake food can usually provide enough nutrients, but it's always more interesting to watch your fish chasing their lunch, while it tries to escape.
Also known as water fleas, these critters are often found at fish stores. They are a good nutrient source, and it's fun to watch the fish chase them as they hop around. If not eaten immediately, they can safely be left to live in the aquarium for a while, until the fish discover them.
These are a kind of worms, which are popular with aquarists. They are a great source of proteins, but be careful about overfeeding - your fish can even get fat on those!
Before giving any worms to fish, make sure that they are still alive. When worms die, they go bad very quickly. Of course, this means that you shouldn't freeze them, too.
There is probably no living thing that could be named a natural enemy of the human beings -- but the mosquitoes are getting close. Anyone that has lived in warmer climates will confirm that.
Anyway, since they have aquatic larvae, in nature tropical fish are the factor that limits their reproduction. That's why many fish like to eat these larvae.
If there's a lake or puddle nearby, you can probably find them in the summer and scoop them up with a coarse fish net. You can also prepare a bucket with garden pond water and some grass clippings and wait for the mozzies to lay their eggs. Be careful, because if some larvae remain uneaten, well, you won't be very popular in the neighbourhood.
It's easy to culture these critters, and fish really like them. Put a few rotting fruits in a jar and leave that in open space. You'll soon find a few fruit flies inside. Just stopper the jar with a wad of filter floss, so the flies can't get out but air can still get in.
When feeding them, a good idea is to put the jar upside down, partially in the water. Occasionally a fly will drop in, and the fish will eat it. It's a good idea to put them into the fridge for a little while to prevent them from being over-active and too adventurous.
Many fish need vegetable food as a supplement, and they will eat almost anything that we do. Many aquarists use spinach or lettuce leaves.
Before putting the veggies into the aquarium, drop them for a few minutes in unsalted boiling water to kill the bacteria and let them sink.
It has to be noted that most fish won't eat it the very moment when it's put in the aquarium. They will, however, devour it after the second or third day.
While you're on a vacation
If you're going to be out of town for a few days, fish may do without any care. Healthy fish can easily go without food for up to a week, and the fasting can actually be good for them.
If you're going to leave for a longer period, there are many fancy ways for automatic feeding, but still the best method is using a helpful friend or neighbour.
The best way to keep an inexperienced friend from overfeeding your fish is putting daily food doses in small envelopes and instructing them to feed the fish one everyday. Tell them not to make up for days they've missed by feeding extra food to the fish.Project: Maintaining a Happy Aquarium