Humans use lungs to pull in oxygen from the air around us; much in this same way fish pull in oxygen from the water, only their "lungs" are called gills and are external. Gills work similarly in freshwater and saltwater fish, however there are some physiological differences in how their surrounding water is processed.
In human lungs, millions of capillaries full of blood run near the surface of tiny air sacs called alveoli. These air sacs take in oxygen from the surrounding air which then transfuses through the capillaries and into the blood. This is how oxygen moves into our blood and throughout our system.
Gills work the same way, except that they use water instead of air to pull oxygen into the blood. Gills also lie on the outside of the fish's body, allowing the process to happen immediately as they move through their habitat.
Similar to our millions of air sacs and capillaries, the skin's surface area is important when it comes to this transfusing: Oxygen transfers through the permeable surface of the skin and into the blood system. The more surface area means the more oxygen can be brought into the blood. Water has 95 percent less oxygen in it than air. To help with the much lower oxygen levels, the surface area of gills are relatively large to the fish's body, allowing them to filter in 80 percent of the oxygen in the water. Compared to our minuscule 25 percent oxygen intake per breath, fish have adapted well to their environments.
Finding Balance in Life
Cell membranes are what allow certain elements --such as oxygen-- to transfer through the surface of the skin. These membranes allow some things to go through, but keeps other things out; this is called "osmosis." Substances that travel through osmosis move from higher concentrations to lower concentrations in an attempt to "balance." High concentrations of water versus salt move toward equilibrium through the skin of fish.
Saltwater fish have high water concentrations inside of them compared to the salt concentrations surrounding them, so water diffuses out through their skin surface. To deal with the water loss from their bodies, they drink constantly, allowing their gills to remove salt and process oxygen from the water. The insides of freshwater fish naturally contain more salt than their freshwater habitat. Because of this, water moves into their bodies instead of out: While they do not need to drink water, they still need to urinate.
If you put a freshwater fish in salt water, the extra salt in the water would absorb the now higher water concentration from the freshwater fish, and it would die from dehydration and excess salinity in its blood. A saltwater fish placed in freshwater would absorb too much water through its gills, and it would die from overhydration. While the gills work in the same way, the physiological adaptations in the kidneys of saltwater fish process water differently, allowing them to survive in their own natural habitats.
Pump It Up
Oxygen reaches water in two methods: surface area and live aquatic plants. Plants release oxygen during photosynthesis, while atmospheric oxygen dissolves into water from the surface. This oxygen is what fish filter through their lungs to breathe.
While plants supply great additional oxygen, surface area is the best source of oxygen for your fish. One way to increase the amount of oxygen found in water, while expelling harmful trace gasses such as carbon dioxide, is to agitate the surface of the water. This increases the area exposed to fresh air, allowing it to infiltrate the tank. Agitation is easily accomplished through water aeration, or aquarium pumps.
Contrary to appearances, the air bubbles in tanks do not bring oxygen directly to the water. Instead, they consistently move the surface of the water at the top of the tank, or -- in the case of waterfalls and filters -- bring oxygen-rich water deep into the aquarium setup, keeping your fish alive, happy and breathing well.