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 Salt-water or Marine Aquarium. As the greater part of the most curious live stock of the salt-water aquarium live upon or near the bottom, so the marine tank should be more shallow, and allow an uninterrupted view from above. Marine creatures are more delicately constituted than fresh-water ones; and they demand more care, patience, and oversight to render the marine aquarium successful. Sea-sand and pebbles, washed clean, form the best bottom for the salt-water aquarium. It must be recollected that many of the marine tenants are burrowers, and require a bottom adapted to their habits. Some rock-work is considered essential to afford a grateful shelter and concealment to such creatures as are timid by nature, and require a spot in which to hide: this is true of many fishes. Branches of coral, bedded in cement, may be introduced, and form beautiful and natural objects, on which plants will climb and droop gracefully.