Some of the blossom-heads may also be sprinkled over the surface, where
they will live and bloom all through the summer. The _Hydrocharis_,
or Frog's-Bit, and the _Alisma_, or Water-Plantain, are also easily
obtained, hardy and useful, as well as pleasing. Many rarer and more
showy varieties may be cultivated; we have given only the most common
and essential. All the varieties of _Chara_ are interesting to the
microscopist, as showing the phenomenon of the circulation of the sap,
or Cyclosis.

Of the living tenants of the aquarium, those most interesting, as well
as of the highest organization, are the fishes. And among fishes, the
family of the _Cyprinidae_ are the best adapted to our purpose; for we
must select those which are both hardy and tamable. _Cyprinus gibelio_,
the Prussian Carp, is one of the best. It will survive, even if the
water should accidentally become almost exhausted of oxygen. It may
be taught, also, to feed from the hand. None of the carp are very
carnivorous. _Cyprinus auratus_, or the Gold-fish, is one of the most
ornamental objects in an aquarium. But the Minnow, _C. phoxinus_, is the
jolliest little fish in the tank. He is the life of the collection, and
will survive the severest trials of heat and cold. The Chub, a common
tenant of our ponds, is also a good subject for domestication. The
Tench and Loach are very interesting, but also very delicate. Among the
spiny-finned fishes, the Sticklebacks are the prettiest, but so savage
that they often occasion much mischief. For a vessel containing
twelve gallons the following selection of live stock is among those
recommended: Three Gold Carp, three Prussian Carp, two Perch, four
large Loach, a dozen Minnows, six Bleak, and two dozen Planorbis. Some
varieties of the Water-Beetles, or Water-Spiders, which the fishes
do not eat, may also well be added. The Newt, too, is attractive and
harmless.

All may go on well, and the water remain clear; but after the tank has
been established several weeks, the inner sides of the glass will show a
green tinge, which soon increases and interferes with the view. This is
owing to the growth of a minute confervoid vegetation, which must be
kept down. For this purpose the Snail is the natural remedy, being the
ready scavenger of all such nuisances. Snails cling to the sides, and
clean away and consume all this vegetable growth. The _Lymnea_ is among
the most efficient, but unfortunately is destructive, by eating ho