Freshwater

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Part II: Fish and How to Care for Them
Osmosis and the salt to water ratio
Figure 7-3:
Osmosis in a
freshwater
fish.
Osmosis is a simple process by which a fish maintains the correct salt to
water ratio in its body (see Figure 7-3). Through osmosis, water molecules
constantly pass through semipermeable membranes in the fish to equalize
the amount of salt to water throughout her body. Osmosis is one of the main
reasons freshwater fish cannot live in saltwater and vice versa. (As with
every other rule, there are a few exceptions to this one.)
Salt (Sodium Chloride) Higher Concentration of
Salts Inside the Body
Lower Concentration of
Salts in the Water
Fish that don’t drink water
The salt concentration in the body fluids of freshwater fish is higher than the
salt content of the water in which they live. For this reason, water is always
being drawn into their cells by osmosis. If fish did not have a means of getting
rid of this excess water baggage, they would burst like a balloon that has
been filled to a point where it has exceeded its air capacity, or resemble the
human body after Thanksgiving dinner.
Water is removed by the kidneys in the form of very dilute urine in freshwater
species. Specialized salt absorbing cells located in the gills move salt from
the water into the blood. Very small amounts of salt that are present in commercial
fish foods also help aquatic species remain in balance. The amount of
salt passed into a fish’s body is so small that it does not require much energy
to get rid of it.