Freshwater

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Part II: Fish and How to Care for Them
Other types of fish have different body shapes that help them in their native
environments. For example, the tapered shape of a discus lessens water friction
and helps the discus conserve its energy as it quickly slips between
obstacles (such as tightly packed roots in its native environment) to catch its
prey. You find this fish living in areas with sunken tree roots and other types
of natural barriers.
Round-shaped aquatic species, such as fancy goldfish, are slow swimmers
and tire quite easily. You find these species living in slow-moving waters.
Fish that are flat on their ventral (bottom) side, such as the cory, spend the
great majority of their time moving along the substrate bed in your home
aquarium.
Taxonomy
Scientists classify fish as such because of their unique traits that separate
them from other animals. Mammals such as whales and dolphins are not
“fish” because they are warm-blooded and are required to come to the surface
in order to breathe. As we know, fish do not need to breathe air because
they have the ability to extract oxygen from the water around them.
Scientists classify animals into large groups that have similar physical attributes.
This system is called taxonomy. Ichthyologists (people who spend their
time studying fish) place aquatic species into several categories based on
physical traits so that they can, for example, differentiate fish from other
aquatic animals.
Scientists classify bony fish as having: a backbone (vertebrae), a small skeleton
that protects and supports body weight and internal organs, fins, rays
made of cartilage or bone, gill respiration, separate sexes, and a brain case.
About 90 percent of the world’s fish are bony. The common fish you find in
your freshwater aquarium are bony fish.
The fish in your aquarium are cold-blooded. All
this means is that their body temperature
depends on the temperature of the water around
Cold-bloodedness
them. Metabolic rate also plays a role in body
temperature — active fish have a slightly higher
body temperature than lethargic fish.