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Balanced Aquarium


The production of oxygen by the photosynthesis of plants in open balanced custom aquarium contributes little, if any, to that consumed by the animal life therein was obtained most directly simply by measuring the amount of oxygen present under different conditions.

No one had ever bothered to do this before, and Breder found that as far as oxygen was concerned, an “over or under saturation returns with extreme rapidity to equilibria” with the air above the water. In other words, the water is practically never under or oversaturated with dissolved oxygen. As soon as the slightest deficiency in oxygen exists in a tank, oxygen from the atmosphere passes into solution to make it up. Similarly, if an excess is produced by plants under the influence of bright light, this quickly passes off into the air.

In fact, one might say: Just try to keep oxygen out! Research workers in fish physiology sometimes want to determine exactly how much oxygen a fish consumes, and to do this, they must measure the oxygen in a sealed container of water before and after a fish has lived in it. The problem is to get a seal that will keep out the atmospheric oxygen during the course of the experiment.

Even 11/2 inches of heavy mineral oil floated on the top of an aquarium’s water will not entirely keep out atmospheric oxygen from above, when the fish begin to use up the gas already dissolved in the water below. Scientists have had to design some complicated apparatus to circumvent this difficulty.

Despite this omnipresence of oxygen, every aquarist has at one time or another seen his fish gather at the surface of their tank, “gaping.” What makes them come to the top, breathing rapidly, seeming to be in some sort of respiratory distress? Not a lack of oxygen, but an excess of carbon dioxide. Compared with oxygen, this gas passes from the water into the air and from the atmosphere into solution much more sluggishly. Consequently, when an excess amount of it appears in an aquarium, it takes an appreciable length of time for it to pass off. On the other hand, Dr. Breder found that in tanks where plants were actively engaged in photosynthesis building up carbohydrates out of water and carbon dioxide and giving off oxygen the carbon dioxide remained far below its equilibrium level with the atmosphere for extended periods.

Plants, then, can and do make an aquarium more habitable for aquatic animals by using up the carbon dioxide that the latter produce carbon dioxide which, as Dr. Breder put it, is “the limiting factor as regards the respiratory gases.” If plants were at work all the time, a tank containing them could support more animals than one without. But at night or on dark days, when they cannot carry on photosynthesis, plants breathe like animals, adding their share of suffocating carbon dioxide to the water. They breathe, of course, in bright light, too, but then their respiration is far outweighed by their photosynthetic activity, and they consume far more carbon dioxide than they produce. Without bright light, however, the presence of plants in a tank theoretically lessens the number of fish that the tank will support. Contrary to general belief, putting plants into an aquarium does not make it possible to keep more fish in it without suffocation taking place.

It has long been known that carbon dioxide in excess can kill fish or man. Both amateur and professional ichthyologists, however, have usually neglected the effects of this gas, assuming that oxygen alone was concerned with the respiration of fish. Whether or not a fish will be asphyxiated depends on the concentrations of both oxygen and carbon dioxide dissolved in the water. The more carbon dioxide present, the greater must be the concentration of oxygen to prevent asphyxiation. The principal reason for this seems to be that small amounts of carbon dioxide increase the efficiency with which the blood of a number of fish can deliver life-sustaining oxygen to the tissues but at the same time sharply decrease the ability of the blood to take on oxygen at the gills. Physiologists call this an “exaggerated Bohr effect.”

The extent to which this effect operates in fish varies greatly from species to species. Trout, several characins, and a number of marine fish have been demonstrated to be quite sensitive to carbon dioxide. Carp, goldfish, and various armored catfish show less sensitivity, and the common bullhead shows hardly any at all. As would be expected, those species known by aquarists and fish culturists to be most easily asphyxiated are the ones whose blood is most affected by carbon dioxide.

It is possible for a fish to be unable to utilize oxygen that is present in ordinarily ample quantities all about it simply because there is too much carbon dioxide present. This must be the physiological explanation why carbon dioxide, and not oxygen, is the critical respiratory gas in an aquarium. As Dr. Breder discovered, there is always sufficient oxygen present, but carbon dioxide may build up to relatively high concentrations, since it is a slow-moving gas and can be produced by the respiration of the tank’s inhabitants at a faster rate than it can escape through the water surface. The fish will then be starved for oxygen even though there is plenty of it around, because they cannot utilize it in the presence of excessive amounts of carbon dioxide.

The reason the aquarist gets along so well, even while working under the wrong premise, is that he is doing the right thing for the wrong reason. For example, when he aerates his tank’s water or circulates it, he is not introducing more oxygen, as he usually believes, but facilitating the escape of carbon dioxide.

“A vessel of water containing plants and animals must be looked upon as a little world,” wrote Edwin Lankester in 1856. We can now just as categorically state that it must not be so considered. Although the physiology of plants and animals in an aquarium is identical with the physiology of those in the world at large, the part they play in the ecology, or bionomics, of their tank is quite different from that taken by the sum total of all life in the earth’s grand economy. In this sense, an aquarium is not at all a microcosm but merely a part of a macrocosm part of a larger world from which it cannot be either physically or ideally separated. No balance could be expected to exist in such an open system. Looked at logically, the idea of a balanced aquarium, as far as respiratory gases are concerned, seems baseless. But then, most myths never made a pretense of being logical.

Does the balanced aquarium exist in any sense whatsoever? Most certainly, so far as the chemistry of the water is concerned. In a well-established freshwater standing aquarium the water remains crystal clear and in a relatively static chemical state. This stability, or balance, can be most clearly brought out by comparing marine and freshwater aquaria. Sea water in which animals are living continuously deteriorates in its ability to support life. That is the principal reason marine aquaria are so much more difficult to maintain than freshwater ones.

This is a slightly edited & updated version of an article originally published on “”



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