After you know about fish food ingredients, the next question : What is the ideal composition for my fishes ?
The ideal composition means that the fish food is fit for your particular fishes' need and not polluting the aquarium water.

First, we should know about our fishes feeding habits. Basically, fish may be divided into 3 main groups :

Carnivorous fish (meat eating)
This group consist of fish that has predatory habits such as arowana, piranha, arapaima, aba-aba, alligator, etc. They must be fed almost exclusively with live fish.

Omnivorous fish
This group of fish can eat everything from smaller fish, worms, staple fish food and also plant matter; such as Cichlid (South American, African cichlids), rainbow, barbus, tetras, etc.
For omnivorous fish, ideal protein levels : 40-45%, fat level : 7-9%, carbohydrate : 30%, fibre : 1-2%
Omnivorous or meat-eating fish have much shorter intestines suited to their easily digested diet which is rich in protein.

Herbivorous fish (plant eating)
This group of fish like to nibble aquatic plants, usually feed on the aquarium bottom; such as loaches (kuhli loaches, botia, corydoras, etc.), plant-eating suckermouth catfish (plecostomus, ancistrus, etc.).
For herbivorous fish, ideal protein levels : 30-35%, fat level : 3-5%, carbohydrate : 45%, fibre : 3-5%
Herbivorous species need more carbohydrates in their diet since carbohydrates are readily available source of energy along with fibres which are vital for their digestive system. Herbivores have long intestines, ideally suited to vegetable diet which is difficult to digest.

For plant-eating suckermouth catfish, particular requirement of this beautiful fish are often overlook. They consume wood fibre which is vital for their digestion to function properly. If there's no wood to eat, they eat plant leaves for the source of fibre.
If you have this kind of fish, find the sinking fish food that contains wood fibre as part of the ingredients.
But not all suckermouth catfish eat plants and fibre, some eat meat-oriented food such as crustaceans, worms, etc. For example: zebra pleco, whiptail catfish, etc.