Fish, Oxygen, Water
Name: John S.
If a fish takes oxygen out of water, where does the hydrogen go?
Fish do not break H2O into hydrogen and oxygen. There is oxygen gas dissolved in the water.
Their gills extract this oxygen gas out of the water and return CO2 just as our lungs do.
You are likely to be deluged with answers to your question. Here is my take on the matter. The
fish does not decompose water into hydrogen and oxygen. The oxygen taken up by the fish is oxygen
gas that is dissolved in the water. Many gases exhibit solubility in water. One with which you are
familiar is carbon dioxide -- the gas that imparts the bubbles and tingle to carbonated beverages.
Sometimes fish fanciers boil tap water before putting it in their aquaria. This is to cause
discharge of any dissolved chlorine that may be present. Gases are much less soluble in hot
water. Sometimes the well-intentioned hobbyist forgets (or neglects) to re-aerate the boiled
and cooled water before pouring it into the aquarium. If the aquarium is not equipped with a
bubbler system to re-introduce air (oxygen) into the water, the fish will die. So, fish survive
because oxygen is water-soluble.
Actually, by percent, there is more oxygen in air than can dissolve in water. That being true,
why can't fish out of water survive? Among other things, it is because the fish gill tissues
are thin membranes that allow oxygen-bearing water to move between the layers and enable oxygen
to diffuse into the fish's bloodstream. When the fish is removed from water, the surface tension
of water draws the layers into contact with each other. This greatly reduces the gill surface
area accessible to oxygen bearing water -- under such circumstances, the gill tissue has only
two sides. Thus, the fish suffocates because its breathing apparatus has been tremendously
compromised by the adherence of the gill layers. This accounts for the practice of catch-and-release
fishermen who (in preparation for the release) sometimes hold a fish in the water while moving it
back and forth. The purpose of this procedure is to create a turbulent flow of water around the
gills, thereby encouraging the gill layers to separate from each other and enable the traumatized
fish to regain its ability to breathe.
Fish use oxygen dissolved in water. They cannot break water molecules apart to get the oxygen.
A few things:
First off air and water are mixtures of different substances. Air is made of mostly nitrogen gas
and oxygen gas and very small amounts of carbon dioxide, methane etc. Air also has gaseous water
molecules and liquid water molecules. Warm air can hold more water than cold.
If you think of liquid water in the same way but being made up overwhelmingly by water it might
help. Mixed in with the water are gaseous molecules of oxygen and small amounts of carbon dioxide
etc. When fish take in water by their gills the oxygen in the water diffuses from the water
through the gills and is bound by the hemoglobin in the blood of the fish...much like our lungs.
Each oxygen that is part of each water molecule is bound by a strong covalent bond to each of the
two hydrogens in a single water molecule. Therefore the oxygen that fish use is derived only from
dissolved oxygen free in the water...none of the oxygen fish absorb through their gills comes from
the tightly bound oxygen in the water molecules.
Part of this misconception comes from people also thinking that the bubbles in boiling water are
oxygen and hydrogen...they are water turning to a gaseous state and rising through the water
because they are less dense than the liquid water.
The fish is taking the oxygen that is dissolved in the water. Just like CO2 is dissolved in
the water that makes up any carbonated beverage. In the pond,the water is the solvent the
oxygen is the solute. The fish is not breaking down water into its components. That would
take an awful lot more energy and would spell disaster for all of us living on this wonderfully
That is also why you would need a bubbler or some plants in your fish tank. They replenish
what the fish use.
Hope this clears things up.
Fish take oxygen (O2) that is dissolved in water that originates in the atmosphere. Do not confuse
this with oxygen produced by the decomposition of water: 2H2O -----> O2 + 2H2. This reaction doesn't
occur "naturally" in water. It only results from an electrochemical or photochemical reaction. In
such reactions, energy in some form is supplied from an external source. The
reason fish tanks have a bubbler to pass air through the water is to replenish the supply of
dissolved oxygen from the atmosphere and to provide some turbulence so that the water keeps
uniformly supplied with oxygen and other nutrients.
Fish do not use the oxygen that is bound in water molecules, they use O2 that is dissolved in the
water. If dissolved oxygen is too low fish will die, which often happens in shallow lakes in
winter - winter kill - or with pollution or other environmental problems.
John, the fish is not taking oxygen out of the H2O molecule itself. It is taking up pure
oxygen (O2) that is dissolved in the water. Gasses can dissolve into a fluid: think about
the CO2 gas that is dissolved in your carbonated soda.
Paul Mahoney, PhD
The oxygen that the fish use in water is dissolved oxygen gas. The oxygen does not come
from the water molecule (H2O). Fish need an average dissolved oxygen concentration of
about 4 to 6 parts per million to stay alive. I hope that this answers your question.
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Update: June 2012