Aquarium Carbon Dioxide
Although plants have been grown in home aquaria for more than a hundred years, their functions in such small bodies of water are still largely misunderstood. Not only aquarists but teachers and scientists as well have entertained incorrect ideas about the part that plants play in the so-called "balanced aquarium" ever since vegetation was first put into the tanks with fish. Strangely enough, this century of error has not handicapped the keepers of home aquaria to any great extent. Being empirically minded, aquarists have accepted and employed practices and techniques that "worked" without concerning themselves too much as to just why they worked. This practical attitude has produced beautiful tanks and healthy fish but has sometimes placed fish fanciers in the comfortable but somewhat strange position of doing the right thing for the wrong reason.
There are perhaps half a dozen different reasons why the aquarist should try to keep plants in his aquarium, but the oxygenation of tank water is not one of them. In fact, the idea that the fish and plants of an aquarium balance each other in their production and consumption of carbon dioxide and oxygen is a false one of such long standing and general acceptance that it perhaps deserves to be called a myth.
The usual concept of the balanced aquarium is quite simple and a little too pat, it turns out. Fish and aquatic plants utilize gases, oxygen and carbon dioxide, that are dissolved in water. Animals, including fish, of course, respire, taking in oxygen and giving off carbon dioxide. Plants also respire, but in the presence of light that is strong enough and of the proper wave lengths they also carry on the process of photosynthesis as a result of which carbon dioxide and water are taken in and oxygen released. In the actively photosynthesizing plant, this process well overbalances that of respiration so that the net result is the consumption of carbon dioxide and production of oxygen.
The respiring animal "burns" carbohydrates and other energy-producing materials and obtains energy. On the other hand, the photosynthesizing plant stores up energy in manufactured food with the aid of green chlorophyll.
It is obvious that these processes counterbalance one another, and it might be thought that this is evidence in favor of the idea that plants balance fish in an aquarium. This reasoning involves one false assumption, however; namely that an aquarium is a closed system, cut off completely from the outside world. To be more specific, the oxygen and carbon dioxide in the air above the water have not been taken into consideration.
Whenever tank water was oversaturated or undersaturated with oxygen to the slightest degree, it very quickly returned to equilibrium with the oxygen in the air above—whether or not plants were present or whether the measurements were taken in bright sunlight or in the dark. Obviously plants could not have been affecting the oxygen content of the water to any significant extent. The oxygen comes in from the air as fast as the smallest deficiency exists in the water. Similarly, surplus oxygen immediately passes off into the atmosphere whenever any excess is present. Except under extraordinary conditions, there is neither any lack nor excess of oxygen in an aquarium.
How is it, then, that fish sometimes seem to "smother" so obviously—in an overcrowded aquarium, for example? The answer is that carbon dioxide causes their distress and eventually kills them. According to experiments made by physiologists, fish can be killed by carbon dioxide even though there is plenty of oxygen present. Carbon dioxide moves in and out of water much more slowly than oxygen. When aquatic plants were actively engaged in photosynthesis, Breder found that the amount of carbon dioxide in tank water remained far below the equilibrium point. In other words, plants can use up carbon dioxide more aquarium more rapidly than it is able to pass off into the atmosphere. Unless the carbon dioxide content is then reduced in some way, the fish are unable to adjust themselves to the excessive accumulation of this gas and may die—even though there is plenty of oxygen all around them. It should be realized that the amount of carbon dioxide dissolved in water is to all intents and purposes independent of the amount of dissolved oxygen, and vice versa.
When fish come gaping to the top of an aquarium, the cause is an excess of carbon dioxide, not a deficiency of oxygen—except under extraordinary conditions when an oxygen deficiency may exist. For instance, a tank that has gone completely "bad" may contain so many bacteria that the oxygen content of its water is kept dangerously low. Even in such an aquarium, however, the accumulation of carbon dioxide must also play a part in causing the distress of fish, since bacteria produce this gas just as do higher plants and animals.
It has been shown in experimental tanks that the greater the carbon dioxide content of water, the higher must be the concentration of oxygen to "offset" its harmful effects. Anything that tends both to lower the oxygen and increase the carbon dioxide will have a doubly detrimental effect, so to speak. High temperature is such a factor, since it decreases the amount of oxygen (and carbon dioxide) that water can hold in solution but increases the rate at which carbon dioxide is produced by speeding up the metabolism of the fish. It is true that less carbon dioxide can be held in solution, but this gas, we must remember, often builds up concentrations above its saturation point, a thing that oxygen does not do. Rising temperatures therefore cause a relative increase in the carbon dioxide content of aquarium water if not an absolute one. Nevertheless, Breder's measurements show that carbon dioxide, not oxygen, is the limiting gas, so far as respiration is concerned, under all ordinary conditions in aquaria.
Carbon dioxide and not oxygen should therefore be the aquarist's concern. When a tank is aerated artificially by air pumps, very little, if any, oxygen is being introduced—since the water is practically saturated with that gas at all times—but the release of carbon dioxide is being facilitated, carbon dioxide that can build up to dangerously high concentrations, since it can be produced more rapidly than it can escape into -the atmosphere. In effect, the myriads of tiny bubbles rising from an aerator increase the surface through which the gas can leave the water. As has been emphasized by a number of experienced fish fanciers, the surface of water exposed to the atmosphere is the all-important factor in a standing aquarium. In calculating the number of fish that a certain tank will comfortably support, the most critical element is the area of the water's surface, not the volume. Another result of aeration may be the circulation of water within the tank, preventing stratification and bringing carbon dioxide-laden water to the surface.
It is often said that keeping plants in an aquarium will enable it to maintain more fish. If plants were actively photosynthesizing at all times, they would keep down the carbon dioxide concentration appreciably, as Breder showed, thus permitting more fish to live in a given tank. But plants continually respire, just as animals do, and it is only in bright light that their respiration is more than offset by photosynthesis with the net result that carbon dioxide is consumed and oxygen produced. At night or on dark days they are not engaged in photosynthesis and produce carbon dioxide and consume oxygen exactly as do fish. At such times the presence of plants theoretically lessens the number of fish a tank will support. The amounts of gas exchanged by plants during respiration are much less than those by animals, however, so it is doubtful whether the consumption of oxygen and production of carbon dioxide by plants are ever of sufficient magnitude to cause aquarium fish any trouble. For this reason the aquarist need hardly ever worry about having his tank too heavily planted. In all probability, a tank would have to be almost completely choked with plants before seeing the effect.
Anyone who wants proof that plants are not essential to any so-called "balanced aquarium" need only remove all of them from such a tank; the fish will show no respiratory distress whatsoever. This simple experiment should convince aquarists that plants do not oxygenate the water in an aquarium.
