on the surface of
the water, and adhering to the sides of the aquarium. Now observe the
ulva closely, and from its edges and surface very fine threads of
silvery bubbles are pouring out and ascending to the surface. In an
hour's time the water will be thoroughly charged with air. We will again
place twelve more shrimps in the aquarium. This time they will live, and
we will have established a _true_ aquarium--an aquarium based on the
self-sustaining principles of nature, wherein _it will not be necessary
to change the water_.

[Illustration: SHRIMP.]

Fish as well as human beings breathe air. Air is contained in all water.
After the shrimps had breathed or used the air contained in the water
several times over, it became unfit to sustain animal life any longer,
and so they smothered: just the same as if a number of people were
placed in a room, and all the doors and windows and ventilators were
sealed up tight, so that no new air could enter. They, too, would
suffocate in a short time and die. All plants living in water are
constantly manufacturing new and pure air for their friends and
companions the fishes, particularly when under the action of sunlight.

The great secret in establishing a self-supporting aquarium is to
establish a natural balance of water, fish, plants, and light, so that
none of these agents are wanting in quantity. For instance, a strong
light is required to cause a healthy development of the plant life, but
not direct sunlight, or the plants will be forced too rapidly, and death
will soon follow. Again, direct sunlight will increase the temperature
of the water to such an extent that many of the fish will die. If the
animal life is in excess of the plant life and the water contained in
the aquarium, the animals will perish for want of sufficient air. Again,
if the aquarium is overstocked with plants, so that they are crowded so
closely that the light fails to reach some of them, decomposition will
take place, and everything will become a decaying mass. In fact, it is
only by beginning on a very modest scale, with a very few and small fish
at first, and by gradually increasing the number, that a beginner can
expect to succeed. Over-stocking with animal life and overfeeding are
the two greatest temptations that beset the path to success for the
aquariumist; but patience, perseverance, and critical observation will
eventually lead to success.

The greatest care must be taken, and all shells, rock