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.
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