specific gravity, as indicated by the hydrometer, is restored.

The aquarium may be found some morning with a deep and permanent green
stain discoloring the water. This unsightly appearance is owing to the
simultaneous development of the spores of multitudes of minute Algae and
Confervae, and can be obviated by passing the water through a charcoal
filter. When any of the fishes give signs of sickness or suffocation, by
coming to the surface and gulping air, they may be revived by having the
water aerated by pouring it out repeatedly from a little elevation, or
by a syringe. The fishes are sometimes distressed, also, when the room
gets too warm for them. A temperature of 60° is about what they require.
And they will stand cold, many of them, even to being frozen with the
water into ice, and afterwards revive.

The degree of light should be carefully regulated by a stained glass
side, or a shade. Yet it must be borne in mind that sunlight is
indispensable to the free evolution of oxygen by the plants. And when
the sun is shining on the water, all its occupants appear more lively,
and the fishes seem intoxicated--as they doubtless are--with oxygen.

A novice is apt to overstock his aquarium. Not more than two
moderate-sized fishes to a gallon of water is a safe rule. Care, too,
must be taken to group together those kinds of creatures which are not
natural enemies, or natural food for each other, or a sad scene of
devastation and murder will ensue.

Cleansing cannot be always intrusted to snails. But the sides may be
scrubbed with a soft swab, made of cotton or wick-yarn. Deaths will
occasionally take place; and even suicide is said to be resorted to by
the wicked family of the Echinoderms.