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Name: John S.
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 8/20/2003

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.

Van Hoeck


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.

ProfHoff 715

Fish use oxygen dissolved in water. They cannot break water molecules apart to get the oxygen.

Tim Mooney

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.

Peter Faletra

Hi John,

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 wet planet.

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.

Martha Croll

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.

Vince Calder

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.

J. Elliott

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

John S.,

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.


Bob Trach

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