Some fish with mouths on the underside of their head, like the catfish, feed on the bottom. The shape and size of a fish's mouth can also tell you what it eats . Since tropical fish in an aquarium have small mouths, Tetra has developed a variety of small fish food flakes, granules and tablets for daily feedings. Breathing Underwater Fish, like most organisms, need oxygen to survive. The oxygen enters the water surface by diffusion or in the water from plants as a byproduct of photosynthesis.
Water enters the fish's mouth, moves across the gills and passes out the gill slits or operculum.
The gills are made up of a bony or cartilaginous arch supporting a large number of paired gill filaments. Beneath the thin membrane is a network of blood vessels. Oxygen diffuses from the water through the membrane into the blood and carbon dioxide diffuses outward . Most fish have well- developed eyes on the sides of their heads. This placement allows them to see in all directions.