Inquisitive minds want to know. How can fish breathe underwater if there is no air? Fish and humans resemble each other in many ways. We have digestive, circulatory and nervous systems similar to those of other vertebrates. And fish need oxygen just like we do. But when we spend time underwater we must carry air with us.

What really makes a fish different is the respiratory system. The water that surrounds a fish contains dissolved oxygen in a ratio of about 5 ml per liter. Therefore they must have a system that can extract that small amount of oxygen out of the water and into their bloodstream. They use their gills for this, which are found between the mouth and the beginning of the gut or food tract called the pharynx.

The process starts with the fish's mouth where he takes in water. When a fish opens and closes his mouth, he is pumping water back through the gills – his own way of breathing. The gills contain thousands of tiny capillaries (blood vessels), so as water passes over the gills, oxygen is absorbed directly into the bloodstream.

Bony fishes, which are the most common, have an effective pumping system that involves the mouth and the outer cover of the gills, called the operculum. When the fish's mouth opens, the operculum closes, drawing water into the mouth. When the fish closes his mouth, the operculum opens and allows fresh water to cross the gills. Oxygen passes through the thin wall of the gills and into the blood.

Other fish, such as the tuna, have less effective pumping systems and must swim constantly to keep fresh oxygenated water flowing over their gills. They usually swim with their mouths partly open.

Their World of Water

Water temperature can greatly affect the concentration of free oxygen in the water. As water temperature increases the free oxygen concentration decreases. Stagnant or poor quality water also contains less oxygen. Since some fish require more oxygen than others, this helps to explain why some fish thrive in specific habitats. For example, trout prefer the cool waters of the northern streams because they are quite active and the cool water retains dissolved oxygen more readily. Carp are sluggish and don't require as much oxygen; they can thrive in warm, relatively stagnant water, such as ornamental ponds.

The surface of the water where it comes into contact with the atmosphere tends to hold a higher content of oxygen. Goldfish can survive in non-aerated aquariums because they spend most of their time at the surface.

The Home Aquarium

Just as fish extract oxygen through their gills, they also produce carbon dioxide as waste and discharge it into the water. To be certain that there is enough oxygen in the water for the fish to "breathe," the carbon dioxide needs to be replaced by oxygen. This is called aeration.

Aeration is the process by which oxygen is transferred from the air to the water. In the aquarium aeration is accomplished by breaking the surface of the water or by creating turbulence in the water. This is relatively easy to achieve in a freshwater tank, a bit more difficult in a saltwater aquarium.

In your fish tank, aeration happens in a few ways:

First, on the surface of the water in the tank, dissolved gases are released into the air and exchanged for oxygen. The greater the surface area, the greater the amount of oxygen taken in, which is why fancy narrow tanks or fish bowls with very little surface area can only support a couple of fish.

Second, when water passes through a power filter and is exposed to the air before it's pumped back into the tank (which is the case with most power filters) the water is being aerated.

Finally, an air pump connected to a bubbler in the tank - an airstone or one of those little bubbling divers - provides air from the bubbles as they burst in the tank. These are all methods of mechanical aeration. Aeration also occurs when live plants release oxygen as they grow.

Since small fresh water tanks - 10 gallons and less - have little surface area, adding aeration through an airstone can be helpful. In larger fresh water tanks outfitted with power filters, the movement of the water caused by the filter will usually provide enough oxygenated water without an airstone, although the stream of bubbles from the stone can be pleasant to watch and certainly won't be doing any harm.

Our fish are not so different from us – they see, taste, smell, hear and breathe – but they do it in their own underwater world. Understanding how fish utilize oxygen and rid their bodies of carbon dioxide, all underwater, will help you provide them with the best care.