Freshwater

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Part II: Fish and How to Care for Them
Many species use the pectoral fins to incubate their eggs with water during
the brooding period. Many flying fish have adapted their pectoral fins into
wings so that they can take short flights through the air. Some species of catfish
can “lock” their pectoral fins into a rigid stance when defending themselves
from predators.
The pelvic fins
Pelvic fins aid fish in braking, stabilizing their bodies, and changing directions.
These fins are located in front of the anal fin on the abdomen of the fish
(one on each side). Other uses of the pelvic fins include searching for food,
carrying eggs, and fighting. These fins are usually smaller in open water
species like the freshwater platy, and larger in some bottom-dwellers.
These fins are often called ventral fins because of their position on the body
near the small body-cavity opening on the bottom of the fish.
The adipose fin
A few species of fish such as tetras and some catfish have an extra fin called
the adipose fin, located on the back between the dorsal and tailfins.
Hobbyists often refer to it as the second dorsal fin. Scientists have not found
any physical reason for this fin to exist. At this point, it has no known use.
But it looks cool, so why not?
Swimming movement
The special body shape of fish helps increase the overall efficiency of their
swimming movement. A fish’s body is usually tapered at the head and tail
and bulky in the middle (like many of us when we hit midlife). This tapering
allows fish to slip through the water without much effort. So, maybe if we can
find a way to live in our bathtub, we’ll have it made.
Looking carefully at your fish, you may notice that most of them swim with
little or no effort, which is surprising because water is much more resistant
than air. But water’s liquid form supports a body’s weight as the body moves.
Because your fish’s weight is suspended in water, she needs only a small
amount of energy to overcome the force of gravity — as opposed to the effort
we humans must put out as we move through atmospheric air on dry land.