Just like humans, fish require oxygen for survival. Fish use oxygen that they
strip from the water and produce carbon dioxide as a waste product. Any
living plants in your aquarium use this carbon dioxide, and eventually expel
oxygen back into the water.
The gill method
Unlike land animals, fish don’t get their oxygen from air. Instead, fish take
their oxygen directly from the water through their gills. Gills are lined with a
large number of blood vessels that help retrieve oxygen.
Gills are very similar in structure and form to human lungs, except that they
are a whole lot more efficient: Although fish remove up to 85 percent of the
oxygen from their aquarium water, humans obtain only about 25 percent of
the oxygen in the air they breathe by comparison, so water quality is important!
(Of course, if you live in a crowded city, your oxygen consumption may
drop to about 2 percent.)
Water enters a fish’s mouth and passes across the gills where the oxygen is
extracted by the gill filaments. The oxygen-depleted water is then quickly
Fish with high energy levels who are very active, like the freshwater danio,
must constantly keep swimming in order to force water through their gills
and obtain oxygen. Species of fish with high energy levels would eventually
suffer asphyxiation if kept in a small aquarium that restricted their swimming
movement. You don’t want to live in a sealed elevator with 20 other people.
Neither do your fish.
Make sure that your tank is large enough to provide ample swimming room
for your aquatic friends.
When moving fish from one location to another, you must remember that gills
are made out of fine tissue that can collapse if removed from water. The gills
are structurally supported by the weight of the water itself. So it is very
important that you keep your fish in water while moving them, to avoid causing
any damage to their gills, bodies, and fins which can be crushed by lack
of water support.
The labyrinth organ method
A certain group of fishes (known as the Anabantids), found in Asia and Africa,
are able to breathe air directly from the atmosphere, using a specialized
organ called the labyrinth. The labyrinth, located inside the head behind the
gills, has evolved over time to take oxygen directly from the air as a supplement
to extracting it from the water.
Anabantids include bettas, gouramis, and paradise fish. In the wild, many of
these fish live in dirty, poorly oxygenated waters full of strange-looking creatures.
(Not unlike a good day at our public pool.) These fish tend to have
wide bodies and enlarged fins.
The physical shape of the labyrinth organ gives rise to its name, which literally
means “maze.” The labyrinth contains rosette-shaped plates that have
thousands of oxygen-absorbing blood vessels, which gather air that is
inhaled. The inhaled air is then trapped inside a group of folds (which resembles
a sponge) and is eventually absorbed into the main bloodstream.
Anabantids can survive in a smaller aquarium space than that which is normally
provided (usually 10 or more gallons) because they can extract oxygen
from the air. However, this does not mean that anabantids can or should be
kept in very crowded conditions or extremely tiny tanks. Even though they
have the ability to breathe “extra” air, these fishes still add as much waste to
the water as their tankmates and need proper space and filtration for healthy
You will often see betta fish for sale in small jars. The reason many dealers do
this is to keep the males separated so that they don’t fight. This is not a good
practice for the home aquartist to pick up and use. Don’t keep labyrinth fish
in small bowls or hanging vases for decoration purposes. Give them a good
healthy aquarium environment with plenty of room instead.
Anabantids can develop diseases brought on by crowded tanks with bad
water conditions, just like other species. Take our word for it, they’ll be
healthier and happier in a proper aquarium. (Anabantids should be provided
with the same high quality filtration, heating, and other proper conditions as
is standard with other tropical fish.) They will like you a lot better too.
Like humans, fishes have five senses: taste, sight, hearing, touch, and smell.
Fish use all these senses to locate food, communicate with one another,
attract mates, and avoid bigger and meaner fish. Fish have been known to
learn to do without one or more of their senses when they’re injured or born
with a physical defect. We’ve seen fish in the worst possible physical condition
continue to survive. Think how great they can look and feel if we keep
them in the best possible condition!
Here are some fun facts about fish eyes:
Most fish have the ability to see in two directions at the same time. This
physical phenomenon is known as monocular vision.
Fish can’t completely focus both their eyes on a single object at the
Fish do not have eyelids and sleep with their eyes wide open, resting in
a hypnotic state.
Most fish are nearsighted and see clearly only about a foot away. So, if
you stand across the room, smiling and wildly waving both your hands
to entertain your fish, don’t hold your breath waiting for them to
The lateral line system
Fish have an interesting system known as the lateral line, which helps them
locate objects in their path and in their surrounding environment that they
cannot see normally due to their limited eyesight (see Figure 7-2). This line is
incomplete in some species. The blindcave fish use this system to navigate,
and killfish use it to help locate insects above the surface of the water.
The lateral line is located on both sides of their body and runs from the back
of the eye to the base of the tail fin. These lines are composed of small neuromasts
(receptors) which contain cilia (very fine hairs) in fluid-filled canals.
These canals detect vibrations in the water, and the vibrations form an
“image” inside the fish’s brain.
A fish’s eyes are often large to compensate for the poor lighting conditions
that exist under water. Usually the eyes are located on the sides of the head,
and some species can rotate them 360 degrees. In certain species that live in
areas of total or semi-darkness (such as the blindcave fish), the eyes are
absent altogether. Over time, the eyes have been selectively removed
through the evolutionary process.
Some fish do have the ability to see a few colors at various depths, but they
have great difficulty adjusting to rapid light changes because their iris works
slowly. For this reason, fish act “shocked” and may panic when an aquarium
light is suddenly turned on or off without warning in a room that is still dark.
So if you turn on your aquarium light right after you get up in the morning
and then notice that your fish are stuck to the ceiling, you probably frightened
them a little bit.
Within the human eye, the shape of the lens is constantly changing in an
effort to achieve proper focus. The lens in a fish’s eye remains the same
shape, but focuses with help from special ocular ligaments that actually
move the eye forward and backward in its socket.
Fish do not have complex ears like we do because sound travels in water
much faster than in air, so by evolution standards this was not needed. Fish
ears are composed of a simple inner chamber. Vibrations picked up from the
environment are passed over sensory components, which generate sound.
Most ichthyologists (fish experts) believe that a fish’s swim bladder works
together with the components of the inner ear to distinguish specific sound
Smell plays an important role in detecting food and prey, and in locating a
suitable mate. Fish take in smells through their nostrils, which are connected
to their olfactory system. This olfactory system is not completely joined with
the respiratory system and acts as a separate unit.
Fish have taste buds on their mouths, lips, and, in special cases, on their fins.
The complete range of taste for fish is very short, so they must constantly
forage through their environment in hopes that they can “stumble across” the
food they need to survive. Catfish have evolved barbels (whiskerlike appendages)
that contain taste buds for locating food in cloudy or dark water.
The old argument as to whether fish can feel pain or not has been at issue for
many years. We would really hate to find out that our fish could feel pain if
we did something that caused them harm. The safest bet is to assume always
that your fish can feel pain and treat them with respect and great care just
like you would any other pet.
Source of information is from, For Dummies. Pictures are from For Dummies, watershed3.tripod.com, mypalhs.com/forums... Basic Google search for Betta labyrinth organ diagram.