Freshwater

Fish that need to drink water
Just in case you decide to keep marine fish after you have become aquainted
with freshwater fishkeeping, you should know that they have the opposite
problem when it comes to balancing water and salt in their body’s cells.
Saltwater fish have a lower salt content in their bodies and must constantly
drink water to replace that lost by osmosis to the saltier environment around
them. If saltwater fish didn’t constantly drink water, they would eventually
die from dehydration.
Saltwater fish excrete small amounts of urine. They also rid their bodies of
excess salt to maintain their overall osmotic balance.
The difference is this: Freshwater fish have high salt content in their bodies.
Water is gained constantly because it comes into their bodies. Excess water
is removed through urine to retain salt/water balance. Saltwater fish have low
salt content in body. Water is lost constantly because it goes out to the saltwater
they live in. These fish drink the saltwater they live in to get the lost
water back into their bodies to retain salt/water balance.
Recognizing Traits to Identify Fish
A fish’s life, habits, and movement are completely dependent upon its overall
body form and size. When you look at the mouth structure and fin design of
each species, you can discover many clues to help answer questions about
the way a fish survives, eats, and moves through water.
Discovering how a fish’s physical form evolved over a long period of time to
guarantee its survival in different aquatic environments helps you recognize
unfamiliar species and can give you an immediate idea of what their aquarium
requirements probably are. Although this is not a scientific rule, it works
about 95 percent of the time.
Body shape
Chapter 7: Fish Anatomy
The specific shape of a fish’s body can tell you about its natural habitat and
swimming range. The streamlined body of a zebra danio, for example, allows
it to slip smoothly and effortlessly through open water with quick bursts of
speed. You won’t find danios living in stagnant swamps in the wild.
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