Different fish food forms & types to meet different fish mouths

Fish with pointing-upwards mouths
This kind of fish are surface fish, with straight backline and downwards curved body characteristic. The mouth position showing that the fish adapted to feed at the water surface, but sometimes they look for food at lower levels. For example: butterfly fish, axe fish.

Surface fish require fish food which floats on the water surface, it can be flake or granulate forms from small to large size depends on the size of your fishes' mouths. Some flakes will sink slowly and can be eaten by fish in the lower levels.

Fish with pointing-forwards mouths
This is the majority kind of fish. They have equally curve body characteristic. These fish live in the middle water levels and feed there. But occasionaly, fish from the middle water level will look for food on the water surface and also the aquarium ground.
For example: rainbow fish, tetras.

For this middle water fish, granulate and tablet forms are suitable for them.

Tablet fish foods can be dropped to the bottom or attached on aquarium wall. While some granulate fish foods are designed either to specifically float or sink, in order to provide food for fish at all water levels at the same time. Some of the granulate floats, whilst the rest sinks slowly to the middle and lower levels.

Fish with pointing-downwards mouths
This kind of fish usually have bearded mouths. They have relatively flat belly line and upwards curved body characteristic. These fish feed on the aquarium ground, searching for food with their beards. Ground feeding fish usually reluctant to look for food at higher levels. For example: suckermouth catfish, loaches (botia, corydoras).

Sinking fish food in form of tablet or granulate are suitable for these kind of fish.