e and good. So
delicate and sensitive are the minute creatures which people the sea,
that they have been found dead on opening a cask in which a new oak
bung was the only source of poison. And no wonder; for a very slight
proportion of tannic acid in the water corrugates and stiffens the thin,
smooth skin of the anemone, like the tanning of leather.

A certain natural density of the sea-water must also be preserved,
ranging between no wider limits than 1026 and 1028. And in the open tank
evaporation is constantly deranging this, and must be met by a supply
from without. As the pure water alone evaporates, and the salts and
earthy or mineral constituents are left behind, two things result: the
water remaining becomes constantly more dense; and this can be remedied
only by pure fresh water poured in to restore the equilibrium. Hence the
marine aquarium must be replenished with _fresh water_, until the proper
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 deg. 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