"Now, if a fish-globe be not overcrowded with fishes, we have only
to throw in a goodly handful of some water-weed,--such as the
_Callitriche_, for instance,--and a new set of chemical operations
commences at once, and it becomes unnecessary to change the water. The
reason of this is easily explained. Plants absorb oxygen as animals
do; but they also absorb carbonic acid, and from the carbonic add thus
absorbed they remove the pure carbon, and convert it into vegetable
tissue, giving out the free oxygen either to the water or the air, as
the case may be. Hence, in a vessel containing water-plants in a state
of healthy growth, the plants exhale more oxygen than they absorb, and
thus replace that which the fishes require for maintaining healthy
respiration. Any one who will observe the plants in an aquarium, when
the sun shines through the tank, will see the leaves studded with bright
beads, some of them sending up continuous streams of minute bubbles.
These beads and bubbles are pure oxygen, which the plants distil from
the water itself, in order to obtain its hydrogen, and from carbonic
acid, in order to obtain its carbon."[A]
[Footnote A:_The Book of the Aquarium_, by Sidney Hibbert.]
Thus the water, if the due proportion of its animal and vegetable
tenants be observed, need never be changed.
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