276 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
It is only when thus arranged that the term aquarium is properly ap- 
plied. 
There is no such thing as an exact balance , to be attained and regu- 
lated by a nice adjustment and the most scrupulous care, as is intimated 
in most works on the subject. On the contrary, there must be simply 
a preponderance of the vegetable or oxygenating element, or, putting 
it more plainly, the animal life must be limited to what will live com- 
fortably in an aquarium under the conditions in which it is placed, 
which are so extremely variable that this limit can only be determined 
by individual observation and experience. All else is mere guess- 
work. 
Oxygen is the element essential to the support of animal life. Car- 
bon is the element essential to the support of vegetable life. Aquatic 
animals in breathing ( i . e., drawing the water through their gills which 
correspond to our lungs) absorb the free oxygen in the water, and also 
decompose the atmospheric air held in suspension in the water and 
absorb or assimilate the oxygen and exhale or throw off carbonic acid 
gas. 
Plants, on the contrary, in growing, under the action of strong light , 
decompose carbonic acid gas — a compound of carbon and oxygen — ab- 
sorb or assimilate the carbon, and liberate the oxygen. Thus there is 
a constant interchange of the elements necessary or vital to each. 
Water is a compound of oxygen and hydrogen, but the water itself is 
not decomposed, and the oxygen, which is a component part of the 
water, is not absorbed or diminished, so that in reality the fishes breathe 
atmospheric air like ourselves and get their supply of oxygen mainly 
from it. 
Now, to show the conditions under which fishes must live in the 
aquarium, let us use human beings as illustrations. An air-tight apart- 
ment containing human beings would be analogous to an aquarium 
tank filled with water alone, and stocked with fish, except that a small 
amount of air would be absorbed by the water in the process of evapora- 
tion. The conditions are nearly enough alike, however, to illustrate 
the point. Any school child twelve years of age should be able to 
point out the fact that as soon as the air in the apartment would be 
breathed over sufficiently to exhaust the oxygen in it the human beings 
would suffocate. This is exactly what would take place in the case of 
the fish. Here, then, comes in the function of the plant life : To renew 
the oxygen absorbed by and essential to the animal life. 
We see, then, that it is absolutely necessary that the aquarium 
should be placed where it will get strong light. Where this is impos- 
sible it is not advisable to attempt to establish an aquarium, as failure 
would be the inevitable result. The alternative is the fish-tank, with- 
out plants and with running water. This advice will apply to those 
who are attracted by the ornamental beauty of the aquarium without 
regard to its scientific aspects, and who think that one would be 
