
THE FRESHWATER AQUARIUM AND ITS MAINTENANCE 

also first applied the term ‘‘ aquarium ”’ to fish tanks, it having previously 
been used by the botanists to designate aquatic plant receptacles. 
THe Property CoNnpbiITIONED oR BALANcED AQuarium. When 
properly arranged, its maintenance is very simple, but the governing prin- 
ciples of a balanced aquarium are not always understood. It is not real- 
ized that when the relations of plant to animal life are correctly propor- 
tioned, the aquarium is virtually self-sustaining or balanced, and the water 
need only be changed at long intervals, often of a year or more. 
Plants in their growth liberate oxygen and take up the carbon-oxide 
and dioxide given off by the living creatures; the latter, in their turn re- 
quire the liberated oxygen, which is either in suspension or dissolved in 
the water, to maintain their existence. Hence, if the plants and animals 
are properly balanced, the quality of the water remains fixed, and only 
becomes vitiated after a long period through the presence of other gases 
generated by the excrement of the animals and the decomposition pro- 
duced by the general decay of the plants, of food, and other organic sub- 
tances. Oxygen is absorbed in considerable quantity by the breathing 
organs of the living inmates, for the oxidation of waste carbonaceous 
matter, thereby producing carbonic acid gas. ‘This, during the daytime, 
is absorbed by the plants, the contained carbon being required to add to 
the solid structure of the plants, and the oxygen isset free in the water. 
-Thus the double action of animals and plants maintains an almost perfect 
balance, as the animals diminish the proportion of oxygen and add to the 
quantity of carbonic acid gas, and the plants increase the oxygen and di- 
minish the carbonic acid gas. 
The fishes, however, consume more oxygen than still water takes up 
from the air, and if oxygen-liberating plants are not introduced into the 
aquarium, they suffer from the lack thereof, become restless, come to the 
surface to breathe the air,and may finally die of asphyxiation. 
It must not, however, be taken that an exact balance is ever attained, 
and it is better to have a preponderance of the oxygenating element, re- 
stricting the animal. life to that which will live comfortably in the existing 
environment, and that the nearer these conditions are approached the 
better the inhabitants will thrive and the less often the water need be 
changed. 
AguariuM Piants. Not all aquatic plants are equally good gener- 
ators of oxygen and some information is requisite to make a selection of 
those which best fulfil this necessary purpose. ‘There are quite a number 
of readily obtainable plants which perform this function, many of them 
native and others to be had of dealers. These are, in the order of their 
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