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 holes
in the young fronds of the larger plants, and thus injuring their
appearance. To this objection some other varieties of snail are not
open. The _Paludina_ and _Planorbis_ are the only kinds which are
trustworthy. The former is a handsome snail, with a bronze-tinted,
globular shell; the latter has a spiral form. These will readily reduce
the vegetation. And to preserve the crystal clearness of the water, some
Mussels may be allowed to burrow in the sand, where they will perform
the office of animated filters. They strain off matters held in
suspension in the water, by means of their siphons and ciliated gills.
With these precautions, a well-balanced tank will long retain all the
pristine purity of Nature.

Specimens for the river aquarium may be readily obtained in almost
any brook or pool, by means of the hand-net or dredge. It will be
astonishing to see the variety of objects brought up by a successful
haul. Small fish, newts, tadpoles, mollusks, water-beetles, worms,
spiders, and spawn of all kinds will be visible to the naked eye; while
the microscope will bring out thousands more of the most beautiful
objects.

A very different style of appearance and of objects distinguishes the