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The position of a fish's mouth gives a general indication of where it feeds in the water column.  In an aquarium, fish with up-pointing mouths like hatchetfish primarily feed on the food flakes that float or hang near the water surface.  Some fish with mouths on the underside of their head, like the catfish, feed on the bottom.  A catfish would be very beneficial to the Aquademics™ aquarium because, as a bottom-feeder, it helps eliminate unused food buildup by eating the food particles that sink to the lower levels of the tank.

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 that fish "breathe" is dissolved in the water.  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.  Numerous small projections with very thin membranes on each filament are the sites of gas exchange (oxygen to carbon dioxide).   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.

Senses

Sight:
Most fish have well-developed eyes on the sides of their heads.  This placement allows them to see in all directions.  The structure of the eye is very similar to that of other vertebrates.  In fish, focusing on near and distant objects is achieved by moving the lens backward and forward by muscular action.  Fish that are colorful can probably see in color.  Bony fish that are active at night have large eyes that help them see in the dark.

Taste:
For fish, taste is more of a close range sense.  Taste buds, which can be found on different parts of the fish, are used in the identification of food.  Taste buds are located in and around the mouth, and, on some fish, on the skin and even on the fins.  Barbels, whisker-like structures near the mouth of fish such as catfish, are covered with taste buds and are used in locating food.

Hearing:
Fish have ears but they do not open to the outside and cannot be seen.   Hearing in fish is well developed, however.  Water carries sound much better than air and the sound waves are picked up through the fish's body.

Lateral Line:
A line of holes runs along both sides of a fish from the head to the tail.   The holes have nerve connections that are sensitive to vibrations.  The lateral line is used form schooling, avoiding obstacles and detecting prey and predators.

Electroreception:
Some bony fish can detect weak electrical fields through pit organs on their heads and faces.  This ability is used for locating prey (all living things give off weak electrical fields), avoiding obstacles in murky water and in migrating.

Buoyancy

Neutral buoyancy, the ability to remain motionless in the water without rising or sinking, is of great importance to a fish.  It allows a fish to spend energy feeding, escaping predators or mating that would otherwise be needed to maintain its position in the water.  Several strategies help achieve neutral buoyancy.   Most bony fish have a structure called a swim bladder, which is a balloon-like organ in the body cavity.  Gases from the blood are added or removed to control buoyancy at different depths.