Freshwater

A fish’s muscle force is achieved though energy created by short fibers that
run throughout the fish’s entire body. These numerous fibers move in
sequence and create physical energy in a series of s-shaped curves. This
energy is then transferred to the tail to provide locomotion. Finally, the
caudal (tail) fin pushes all the water surrounding it backwards, which in turn
propels the fish’s body in a quick forward motion. This sequence of events
allows the fish to move through the water without creating any turbulence —
which would slow it down.
Respiration
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
Chapter 7: Fish Anatomy
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
discarded.
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.
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